feat(#1689): per-plan agent_hint executor routing (#3417)

* feat(#1689): per-plan agent_hint executor routing

Option A per-plan specialist routing: a plan with an `agent_hint:` frontmatter field is dispatched to that subagent instead of gsd-executor when it resolves on the active runtime; absent/unresolved/disabled falls back to gsd-executor (byte-identical). Default-on via workflow.agent_hint_routing.

- src/phase.cts: parse agent_hint into the plan-index JSON (plan_json.agent_hint)
- agent-install-check.cts: resolveAgentHint() reuses getAgentsDir + runtime filename variants; probes project + global agent dirs; fails closed; rejects path-traversing names
- gsd-tools.cjs: 'resolve-agent' query route (fail-closed to gsd-executor; --raw/--json)
- execute-phase.md: lean per-plan reference + {EXECUTOR_TYPE} placeholder (host stays under the ADR-857 Phase 6 byte ceiling)
- execute-phase/steps/per-plan-executor-routing.md: resolution logic (Agent()-based dispatch; advisory on orchestrator-worktree)
- config: workflow.agent_hint_routing (validKey, default-on via SCHEMA_DEFAULTS, boolean validator)
- docs (CONFIGURATION.md, plan-md.md), changeset, tests/agent-hint-routing-1689.test.cjs (17 tests)

* chore(#1689): backfill changeset PR number (#3417)

* chore(#1689): regenerate install-tree fixtures for new workflow fragment

* chore(#1689): ack deliberate execute-phase.md growth (agent_hint routing)

* test(#1689): SPAWN contract allows parameterized subagent_type placeholder

agent-frontmatter's spawn-type checks scanned subagent_type="..." as a
concrete agent name. execute-phase now uses subagent_type="{EXECUTOR_TYPE}"
(a runtime placeholder resolved via resolve-agent, default gsd-executor).
Skip {TOKEN} placeholders in both the known-type and <available_agent_types>
checks; execute-phase still lists the built-in roster incl. gsd-executor.

* fix(#1689): CI conformance for the routing fragment

- per-plan-executor-routing.md: add the canonical runtime-launcher preamble to
  its gsd_run block (runtime-launcher-parity #373), matching sibling step fragments.
- agent-install-check.cts: drop a literal ~/.claude/agents path from the
  resolveAgentHint JSDoc so it does not leak into the compiled engine .cjs
  (cline install leak guard).

---------

Co-authored-by: sim <sim@local>

tests/fixtures/install-tree/opencode.json

@@ -339,6 +339,7 @@
   "gsd-core/workflows/execute-phase/steps/executor-isolation-dispatch.md",
   "gsd-core/workflows/execute-phase/steps/gap-closure-artifacts.md",
   "gsd-core/workflows/execute-phase/steps/partial-wave.md",
+  "gsd-core/workflows/execute-phase/steps/per-plan-executor-routing.md",
   "gsd-core/workflows/execute-phase/steps/per-plan-worktree-gate.md",
   "gsd-core/workflows/execute-phase/steps/post-merge-gate.md",
   "gsd-core/workflows/execute-phase/steps/regression-gate-run.md",