fix(#853): gate manager/autonomous background dispatch by runtime (#863)

* fix(#853): gate manager/autonomous bg dispatch by runtime

/gsd-manager and /gsd-autonomous --interactive dispatched Plan/Execute
via Agent(run_in_background=true). On Claude Code a backgrounded agent
has no Agent/Task tool, so it cannot spawn the nested subagents those
pipelines need — per-plan worktree-isolated executors, the plan-checker,
and the verifier. The phases reported complete but isolation and
independent verification silently never ran, even with use_worktrees /
plan_check / verifier enabled.

Both workflows now resolve the runtime (config-get runtime, default
claude) before dispatching: run plan/execute INLINE on Claude Code so
the nested pipeline runs, and background-dispatch only on runtimes where
a backgrounded agent can still nest. Mirrors execute-phase.md's existing
Codex fail-closed precedent. Reconciles the stale unconditional
background/overlap/lean-context claims elsewhere in both workflows and
in the docs. Adds a content regression test pinning the gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(#853): add changeset for runtime-gated bg dispatch

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/COMMANDS.md

@@ -546,7 +546,7 @@ Interactive command center for managing multiple phases from one terminal.
 **Behavior:**
 - Dashboard of all phases with visual status indicators
 - Recommends optimal next actions based on dependencies and progress
-- Dispatches work: discuss runs inline, plan/execute run as background agents
+- Dispatches work: discuss runs inline; plan/execute run as background agents on runtimes that support nested background dispatch, or inline on Claude Code
 - Designed for power users parallelizing work across phases from one terminal
 - Supports per-step passthrough flags via `manager.flags` config (see [Configuration](CONFIGURATION.md#manager-passthrough-flags))