feat(capabilities): add claude-orchestration capability (Workflow backend) (#1143)

Default-off, BETA, claude-only capability adopting Claude Code's Workflow tool
(/effort ultracode, Agent SDK >= v0.3.149) as an optional parallel-execution
backend for the GSD loop. Restores the wave parallelism + plan-checker + verifier
that #853 forces inline on Claude Code, and folds gsd-ultraplan-phase under one
runtime gate.

- Pure fail-closed core (src/claude-orchestration.cts): detectWorkflowBackend
  (gate ladder: enabled -> Claude -> backend != inline -> nested+background host
  -> valid Agent SDK -> SDK >= floor; every miss degrades to inline) and
  emitWorkflowScript (waves -> parallel() barriers, plans -> gsd-executor +
  worktree, files_modified overlap -> separate stages, resumeFromRunId, budget).
  All interpolated identifiers validated script-safe; briefs JSON-quoted.
- claude-orchestration command family (gsd-tools claude-orchestration
  detect-backend|emit-workflow) for orchestrator invocation.
- Two gated loop contributions at wired points (execute:wave:post, plan:post);
  federated config keys (enabled/execution_backend/min_agent_sdk_version).
- ADR-1143 implementation amendment; CONTEXT.md glossary entry; explanation doc.

On any runtime lacking the Workflow tool, behaviour is byte-identical to today.

closes #1143

docs/reference/capability-matrix.md

@@ -44,7 +44,7 @@ Core package and are stamped with the package version at release (per
 ADR-1244 D6). They are not subject to the consent or integrity-pin flow applied
 to third-party capabilities.
 
-### Feature capabilities (role: feature) — 18
+### Feature capabilities (role: feature) — 19
 
 Feature capabilities extend what the loop does — contributing research,
 planning, execution, verification, or ship artefacts at the loop extension
@@ -55,6 +55,7 @@ points.
 | `ai-integration` | feature | full | `>=1.6.0` | `plan:pre` | step | first-party |
 | `assumption-delta` | feature | full | `>=1.6.0` | `plan:pre` | contribution | first-party |
 | `audit` | feature | full | `>=1.6.0` | — | — | first-party |
+| `claude-orchestration` | feature | full | `>=1.7.0` | `plan:post`, `execute:wave:post` | contribution | first-party |
 | `code-review` | feature | full | `>=1.6.0` | `execute:post` | step | first-party |
 | `drift` | feature | full | `>=1.6.0` | `plan:pre`, `execute:wave:post` | gate | first-party |
 | `external-job` | feature | full | `>=1.7.0` | `plan:post`, `execute:wave:post` | contribution | first-party |