fix: bind backend overrides to exact-task authority

.agents/skills/firstmate-coding-guidelines/SKILL.md

@@ -44,7 +44,7 @@ Every contract - a data format, a state machine, a decision procedure - is state
 Every other mention of it is a one-line cross-reference, never a restatement.
 A single deliberate one-line reinforcement at a genuine risk point is allowed, for example a "don't forget X" placed exactly where forgetting X is costly.
 Restating the contract's substance a second time is not allowed: the two copies will drift the moment only one is edited.
-When you touch a contract, grep the repo for its other mentions and update the cross-references, not duplicate the change into a second full copy.
+When you touch a contract, patch, replace, or prune the owner's existing language rather than appending a new clause or paragraph wherever possible, then grep the repo for its other mentions and update the cross-references, not duplicate the change into a second full copy.
 
 ## Inline-stub pattern
 
@@ -61,7 +61,7 @@ Apply the decision tree above to every line you are about to add to `AGENTS.md`.
 If an addition needs more than a few lines of conditional detail (detail that matters only in a specific situation) or reference detail (a wire format, an exact schema, historical rationale), you are almost certainly adding it to the wrong file.
 `AGENTS.md`'s token cost is paid by every session of every fleet member, every time, whether or not that session ever hits the situation the new lines describe.
 A skill's cost is paid only by the sessions that actually load it.
-When in doubt, write the fact into the skill or doc first, and add only the one-line trigger to `AGENTS.md`.
+When in doubt, write the fact into the skill or doc first by patching that owner's existing language, and add only the one-line trigger to `AGENTS.md`.
 
 ## Trigger hygiene
 

AGENTS.md

@@ -181,7 +181,7 @@ The generic effort fallback and its precedence are owned by `harness-adapters`:
 Do not add model-specific versions of that policy.
 
 `secondmate-provisioning` owns secondmate harness pins and inherited local material, while `harness-adapters` owns the harness consequences.
-Dispatch only on a backend that `fm-spawn` validates as spawn-capable.
+Dispatch only on a backend that `fm-spawn` validates as spawn-capable; pass an explicit per-spawn `--backend` only under that exact task's own authority, never as later-task precedent (selection contract: [`docs/configuration.md`](docs/configuration.md) "Runtime backend").
 A missing dependency, authentication failure, unsupported backend, or version refusal is a blocker; never silently retry on another backend.
 
 ## 5. Recovery

bin/fm-spawn.sh

@@ -10,10 +10,12 @@
 #   installed CLIs were verified to support that axis; unsupported axes are omitted
 #   from that harness's launch rather than guessed.
 #   --backend <name> is the explicit runtime session-provider backend for this
-#   spawn. Without it, the script resolves FM_BACKEND, then config/backend, then
-#   runtime auto-detection (the runtime firstmate itself is executing inside -
-#   $TMUX, HERDR_ENV=1, or cmux runtime signals; bin/fm-backend.sh's
-#   fm_backend_detect, with cmux fallback details in docs/cmux-backend.md),
+#   exact task only (docs/configuration.md "Runtime backend" owns when that flag
+#   is authorized). Without it, the script resolves FM_BACKEND, then
+#   config/backend, then runtime auto-detection from the runtime firstmate's
+#   environment: $TMUX, HERDR_ENV=1, or cmux runtime signals (via
+#   bin/fm-backend.sh's fm_backend_detect, with cmux fallback details in
+#   docs/cmux-backend.md),
 #   then tmux.
 #   Spawn-capable backends are the reference tmux adapter and experimental
 #   herdr, zellij, orca, and cmux. Orca owns both the task worktree and

docs/architecture.md

@@ -109,7 +109,7 @@ All are harness-scoped rather than a global pattern union, and none is a recorde
 The runtime backend is the session-provider layer below firstmate's scripts.
 It owns task endpoint creation, bounded capture, text/key sends, current-path reads for spawn-time worktree discovery when the backend does not create the worktree itself, live-window fallback lookup, agent-process liveness probes where verified, and endpoint teardown.
 `bin/fm-backend.sh` centralizes backend selection, `state/<id>.meta` helpers, metadata-only cleanup identity validation, selector resolution, and operation dispatch; `bin/backends/tmux.sh` is the verified reference adapter ([`docs/tmux-backend.md`](tmux-backend.md)), and `bin/backends/herdr.sh` (P2), `bin/backends/zellij.sh` (P3), `bin/backends/orca.sh` (P4), and `bin/backends/cmux.sh` (P5) are experimental task-spawn adapters.
-New spawns select a backend from `--backend`, then `FM_BACKEND`, then local `config/backend`, then runtime auto-detection from `$TMUX`, `HERDR_ENV=1`, or cmux runtime signals, then default `tmux`.
+[`configuration.md`](configuration.md#runtime-backend-configbackend--fm_backend) owns new-spawn backend selection precedence and authorization.
 Runtime auto-detection is innermost-first: `$TMUX` wins over `HERDR_ENV=1`, which wins over cmux's primary `CMUX_WORKSPACE_ID` marker and documented fallback signals; auto-detected herdr or cmux prints a one-time opt-out notice, auto-detected tmux stays silent, and zellij and orca are never auto-detected (only explicit selection).
 Unknown backend names fail loudly.
 For compatibility, default tmux tasks do not write `backend=tmux`; every reader treats a missing `backend=` field as `tmux`.

docs/configuration.md

@@ -51,7 +51,7 @@ The file format is unchanged in both modes; tasks-axi and manual edits produce t
 For spawn-capable adapters, the runtime session-provider backend controls where task windows/endpoints are created, captured, sent to, watched, and killed.
 `tmux` is the verified reference backend (see [`docs/tmux-backend.md`](tmux-backend.md)); `herdr`, `zellij`, `orca`, and `cmux` are experimental spawn backends (see [`docs/herdr-backend.md`](herdr-backend.md), [`docs/zellij-backend.md`](zellij-backend.md), [`docs/orca-backend.md`](orca-backend.md), and [`docs/cmux-backend.md`](cmux-backend.md)).
 Treehouse remains the worktree provider for tmux, herdr, zellij, and cmux, since herdr, zellij, and cmux are session providers only; Orca provides both the task worktree and terminal endpoint.
-New spawns choose the backend in this order: an explicit `--backend` flag firstmate passes when it spawns a task, then `FM_BACKEND`, then the first non-empty line of local gitignored `config/backend`, then runtime auto-detection from `$TMUX`, `HERDR_ENV=1`, or cmux runtime signals, then default `tmux`.
+New spawns choose the backend in this order: an explicit `--backend` flag that current authority for that exact task alone has authorized (a present captain instruction or the task's own accepted brief; never later-task precedent by analogy), then `FM_BACKEND`, then the first non-empty line of local gitignored `config/backend`, then runtime auto-detection from `$TMUX`, `HERDR_ENV=1`, or cmux runtime signals, then default `tmux`.
 If more than one runtime marker is present, detection resolves innermost-first: `$TMUX` is checked before `HERDR_ENV=1`, which is checked before cmux's primary `CMUX_WORKSPACE_ID` marker and its documented fallback signals - tmux or herdr started from inside a cmux terminal is the innermost, currently-executing layer, while cmux itself (a terminal application, not a nestable multiplexer) is always checked last.
 See [`docs/cmux-backend.md`](cmux-backend.md#runtime-detection) for why cmux can be selected when `CMUX_WORKSPACE_ID` is absent.
 Auto-detected herdr or cmux prints a stderr notice naming `config/backend` and `--backend tmux` as opt-outs; auto-detected tmux stays silent to preserve existing default behavior.