fix(#921,#922): orchestrators must not fork; plan-phase Agent gate is attempt-based (#926)

`context: fork` strips the `Agent` tool from a subagent's environment.
Spawning orchestrators (`/gsd-autonomous`, `/gsd-execute-phase`,
`/gsd-plan-phase`) depend on `Agent` to dispatch sub-agents; running
them forked silently disables the core capability they exist to provide
(#921). Remove `context: fork` from all three command frontmatter files.
`effort: xhigh` (introduced by #769) is preserved.

The `<runtime_compatibility>` Agent-availability guard added by #913 was
checking whether `Agent` was present *before* attempting the call. On
runtimes where the tool list is dynamically resolved this produced
false-negative aborts in sessions that have the tool (#922). Replace
the introspection-based pattern with an attempt-based gate: always
attempt the `Agent()` call; stop only if a real tool-unavailable error
is returned. This preserves #853's backgrounded-session close-off and
false negatives.

Tests updated: enh-769-context-fork-effort.install.test.cjs asserts the
three orchestrators lack `context: fork` and that the converter still
passes the field through for non-orchestrator commands; plan-phase-drift-
guard.test.cjs adds four assertions for the attempt-based gate language;
workflow-size-budget unchanged (budgets not exceeded).

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit b866b95296)

commands/gsd/autonomous.md

@@ -2,7 +2,6 @@
 name: gsd:autonomous
 description: Run all remaining phases autonomously — discuss→plan→execute per phase
 argument-hint: "[--from N] [--to N] [--only N] [--interactive]"
-context: fork
 effort: xhigh
 allowed-tools:
   - Read

commands/gsd/execute-phase.md

@@ -2,7 +2,6 @@
 name: gsd:execute-phase
 description: Execute all plans in a phase with wave-based parallelization
 argument-hint: "<phase-number> [--wave N] [--gaps-only] [--interactive] [--tdd]"
-context: fork
 effort: xhigh
 allowed-tools:
   - Read

commands/gsd/plan-phase.md

@@ -2,7 +2,6 @@
 name: gsd:plan-phase
 description: Create detailed phase plan (PLAN.md) with verification loop
 argument-hint: "[phase] [--auto] [--research] [--skip-research] [--research-phase <N>] [--view] [--gaps] [--skip-verify] [--prd <file>] [--ingest <path-or-glob>] [--ingest-format <auto|nygard|madr|narrative>] [--reviews] [--text] [--tdd] [--mvp]"
-context: fork
 effort: xhigh
 allowed-tools:
   - Read