feat(harness): add plan-level closed-unmerged scenario (AB#3239)

[PolyphonyRequiem]

Summary

First path-coverage scenario for plan-level.yaml. Closes the immediate
acceptance criterion of AB#3239 ("at least one passing scenario exercising
a plan-level sentiment-loop path"). Lands the simplest such path — the
closed-unmerged abort — and establishes the input/cli-stub baseline that
follow-up scenarios (analyzer → fixer → merge loop, stale-generation
re-architect, complete-path child recursion) can build on.

Path covered

depth_guard
  → guidance_loader
    → root_resolver
      → type_loader
        → ancestor_chain
          → state_detector (awaiting_review)
            → pr_poll_platform_router (platform=github)
              → poll_status (route=abort_unmerged)
                → closed_unmerged_gate → $end

The scenario stubs the seven entry-chain CLI verbs (plan depth-guard,
plan load-agent-guidance, root resolve, plan load-type, plan derive-ancestor-chain, plan detect-state, pr poll-status) with the
minimum JSON each agent's Jinja templates inspect. pr_poll_platform_router
is a pwsh inline script that runs unintercepted and routes on
workflow.input.platform.

Single-pass abort — no shim sequencing required.

Verification

• python -m driver.run tests/harness/scenarios/plan_level_closed_unmerged_acknowledges_via_gate → passed: true (event_count=37, gates_presented=[closed_unmerged_gate]).
• Invoke-Pester tests/harness/run-scenarios.Tests.ps1 → 11/11 green (10 existing + 1 new) in ~3m30s.

Scope notes

This PR ships the closed-unmerged abort path only. The richer sentiment-loop
scenarios listed in AB#3239's "Scenarios to add" section remain in scope for
follow-up PRs but are intentionally not bundled here — each one needs its own
agent_scripts (architect, plan_reviewer, pr_feedback_analyzer, pr_fixer) and
in some cases shim sequencing, which makes them substantially larger reviews.
Splitting them out keeps each change reviewable.

Refs AB#3239.