feat(a2a): configurable agent launchers — process and versioned Pi RPC transports

[kidclone3]

Summary

Adds configurable external agent launchers to the A2A inbound server. A non-root served A2A route can now run a configured subprocess or a versioned Pi-compatible RPC worker — safely, with bounded output, process-tree reaping, cancellation, and exactly-once finalization — without changing A2A semantics or breaking existing Hermes routes.

Builds on the A2A platform plugin landed in #77109. Addresses gaps surfaced in #56435 (completion visibility), #78007 (timeout alignment), and #77526 (multi-turn continuity) by providing a first-class external execution path.

What it does

• Process launcher (transport: process): executes one owned subprocess per turn with shell=False, minimal environment, explicit cwd, positive timeout, bounded stdout/stderr (4 MiB), and reply capped at 1 MiB. Supports text and JSON output, optional session-ID extraction, and three continuity modes (stateless, deterministic-key, opaque-session).
• Versioned RPC launcher (transport: pi_rpc): maintains persistent workers per (agent_slug, context_id) using strict UTF-8 LF-delimited JSON frames. Two code-owned profiles — omp (requires ready, settles on agent_end) and feynman (no ready, settles on agent_settled). Blocking extension UI receives correlated cancellation; workers serialize turns, retain early frames, and evict on rejection/crash/timeout/malformed input.
• Common exactly-once finalization: every launcher outcome — whether success, timeout, cancellation, crash, or idle expiry — returns through the existing A2A finalizer for persistence, audit, metrics, push delivery, and task-store writes.
• Backward compatible: root/default requests remain the live Hermes gateway session; profile-only routes without a launcher preserve legacy behavior. Invalid external routes are omitted from cards/health without blocking root or other valid routes.

Changes

File	Purpose
plugins/platforms/a2a/launchers.py	Process launcher: spec parsing, argv expansion, subprocess dispatch, output parsing, session persistence
plugins/platforms/a2a/pi_rpc.py	Versioned RPC transport: worker lifecycle, frame demux, profile dispatch, termination
plugins/platforms/a2a/adapter.py	Route launcher extraction, LauncherManager integration, common finalization boundary
tests/plugins/test_a2a_launchers.py	29 tests: deterministic helper-process coverage for argv isolation, streams, parsing, failures, cancellation, environment policy, session persistence
tests/plugins/test_a2a_plugin.py	Extended with launcher integration tests (108 total, 11 deselected)
website/docs/user-guide/messaging/a2a.md	Operator docs: precedence, schemas, placeholders, environment, continuity, failure mapping
cli-config.yaml.example	Annotated YAML examples for process and RPC launchers

Safety invariants

• External prompts, peers, route slugs, task IDs, and context IDs remain data — never interpreted by a shell
• External children receive a minimal explicit environment, not the gateway credentials
• Timeout, cancellation, failure, and adapter disconnect terminate and reap the owned process tree
• Every operation retains immutable agent, tenant, peer, task, context, and captured profile-home ownership
• Completion, timeout, cancellation, crash, idle expiry, and disconnect races yield at most one visible terminal result

Related

• Feature: A2A (Agent-to-Agent) Protocol Support — Remote Agent Discovery, Communication & Interoperability #514 — Original A2A roadmap (closed)
• feat(a2a): Agent-to-Agent protocol plugin, A2A v1.0 (closes #514) #77109 — Landing PR for the A2A plugin (merged)
• feat(a2a): surface task completions through the user-visible notification path #56435 — Surface task completions through user-visible notification path
• A2A: long tasks can never complete — client timeout (120s) dies before the server reply window (300s), raw-URL calls can't raise it #78007 — Long tasks cannot complete because client timeout is shorter than server timeout
• feat(a2a): inject conversation history on context resume (multi-turn) #77526 — Inject conversation history on context resume