Task: Integrate stuck session settlement fix

Identity

Integrate source commit f1fd13a from task 20260901-fix-session-settlement-a47d2e91 into local main.
Promote the verified missing-settlement recovery behavior into the integrated project snapshot without changing the accepted ADR-006 runtime boundary.
Re-run focused runtime tests, typecheck, and the Renderer/Main/Preload build on the integrated tree, then confirm the development desktop process reloads.

Preserve normal agent_settled authority, per-Conversation generation isolation, exactly-once ownership cleanup, and sibling availability.
Do not replay an accepted or uncertain Coding mutation and do not settle an ordinary prompt solely because message_end was observed.
Permit bounded fallback completion only from exact target-thread idle evidence; contradictory terminal state must converge to a target-only protocol failure.
Keep Pi wire/runtime state private to Electron Main and make no unrelated product, protocol, authentication, provider, or project-data change.

Committed the completed read-only live-stall diagnosis on main as dedbc85 and released its prior feature-task ownership cleanly.
Merged source fix f1fd13a into main without conflicts as merge e76a1e1.
Kept the source task record read-only on its feature branch; main contains the integration task record and canonical promoted facts rather than a mutable copy of another task's owned record.
Promoted the bounded missing-settlement recovery into current-state.md, the evidence index, and README.md; ADR-006 remains unchanged because the fix restores its existing authority, isolation, and no-replay contract.
Restarted the development desktop application from the integrated tree. The new Electron Main process is listening on the Host API and the Renderer returns HTTP 200 with the settlement-probe implementation present in the built bundle.

Source task evidence: 68 focused tests passed across 8 files; 1,732 full unit tests passed and 3 skipped; typecheck, targeted/full lint, Vite build, and git diff --check passed.
Integrated tree: vitest run tests/unit/pi-worker-pool.test.ts tests/unit/pi-agent-server-process-real.test.ts --maxWorkers=2 passed 26 tests across 2 files.
Integrated tree: pnpm run typecheck passed.
Integrated tree: pnpm run build:vite passed for Renderer, Main, Preload, and utility bundles; only pre-existing Browserslist/import/chunk-size warnings remained.
Restart smoke: Renderer http://localhost:5173/ returned HTTP 200; Electron process 87870 listens on 127.0.0.1:13210; the built Main bundle contains the settlement-probe implementation.
check_project_docs.py, task-aware check_doc_drift.py, git diff --check, and the final Git cleanliness check passed.

No additional code follow-up is required for this local integration. A future packaged release should retain the existing Pi final-artifact and cross-platform release gates.