Restore orchestration discipline: orchestrator does terminal fix+PR itself

The first CI pilot run completed green but produced NO PR. Root cause: the
orchestrator delegated the terminal fix step to a background "fixer" sub-agent,
then ended its turn before that agent (and the PR) completed -- exactly the
session-killing pattern the older workflow had guardrails against. Separately,
per-sub-agent model routing turned out to be cosmetic in the gh-aw sandbox: the
api-proxy token-usage log showed every call was claude-sonnet-4.6 regardless of
the requested per-role model.

Fixes:
- Pin the run model: explicit engine.model claude-sonnet-4.6 (drops the
  GH_AW_MODEL_AGENT_COPILOT override; orchestrator + every sub-agent run on it).
- Drop cosmetic per-role routing and the routing-report requirement; document
  per-role routing as a local-only skill feature.
- Terminal fixes are now the orchestrator's own FOREGROUND work (A4/R4); the
  only sub-agents launched are the pass-A writer and the reviewers. No terminal
  background agent. Synthesis is the orchestrator's job (no synthesizer agent).
- Restore the tight escape hatch: once reviewers report, timebox fixing to
  ~10 min, then validate + open the PR.
- Add an explicit "No terminal background agent" Critical rule and simplify the
  completion gate (create_pull_request/noop is the orchestrator's own job).

Recompiled auto-api-docs-writer.lock.yml.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mattleibow