docs(agents): trim section 7 spawn duplication to its owners

[zachlandes]

Trim section 7 Spawn duplication to its owners

Continues the AGENTS.md diet (after #253/#255/#258) with one surgical one-owner consolidation, in the same style as those PRs.
Section 7's Spawn subsection had grown to restate mechanics that already have canonical owners; this replaces each restatement with a pointer, per the one-owner rule in firstmate-coding-guidelines.

Measured size delta

Always-loaded AGENTS.md is paid by every session of every fleet member, every turn, whether or not that session hits the situation the lines describe.

Surface	Before	After	Change
AGENTS.md (o200k_base tokens)	26,759	26,397	-362 (-1.35%)
AGENTS.md (lines)	939	936	-3

What changed (all in section 7 Spawn)

• Runtime backend resolution - the selection order, the innermost-first auto-detection nesting, and the per-backend refusals (including codex-app as unknown) were restated nearly verbatim from docs/configuration.md "Runtime backend".
  Replaced with a pointer.
  Firstmate never computes backend resolution at spawn - the script does - and the actionable "a backend refusal is a blocker, never silently retry another backend" rule already sits on the very next line, so nothing firstmate acts on was lost.
• Per-backend workspace scoping (herdr per-home workspaces, zellij single shared session) was likewise owned in full by that doc.
  Replaced with a pointer.
• The <id>.meta field enumeration (harness=/model=/effort=/kind=/mode=/yolo=, secondmate home=/projects=, and the non-default backend= note) duplicated section 2's canonical <id>.meta line.
  Replaced with a section 2 pointer.
• The grok per-task turn-end hook mechanics duplicated docs/configuration.md (the $GROK_HOME/hooks/ install plus .fm-grok-turnend pointer, teardown removal) and section 2's grok-turnend-token line.
  Removed: harness-adapters already delegates per-task turn-end hook mechanics to bin/fm-spawn.sh, and the hook is installed by spawn and removed by teardown with no firstmate action in between, so there is no wake-time trigger to preserve inline.
• A redundant "backend=codex-app is unknown" line already stated in the spawn command block, section 2, and the runtime-backend pointer.

Safety

• No hard rule, no section 8 always-on supervision core-loop content, and no wake-time trigger was touched.
• Every distinctive token removed (env vars, flags, thresholds, paths) was programmatically confirmed to survive in its owner (docs/configuration.md or AGENTS.md section 2).
• All 14 section numbers and titles are unchanged; CLAUDE.md -> AGENTS.md and .claude/skills -> .agents/skills symlinks untouched.
• Validated locally through the no-mistakes pipeline: review, test, document, and lint each returned zero findings.

Scope note

This is deliberately one surgical theme kept independently reviewable, not a whole-file diet.
Judgment-heavy extractions that need a maintainer decision on shape (the section 14 X-mode cadence block, and the section 7 no-mistakes axi status validation state table) were intentionally left out of this PR.

[zachlandes]

Superseded by #447 (8cd90fe), which removed every line this PR targeted and trimmed further — section 7 now points at bin/fm-spawn.sh's header as the single owner. Closing as redundant.