fix: phantom sessions on reconnect + ForceComplete consolidation + health monitoring skill

[PureWeen]

What

Two reliability fixes discovered during live monitoring, plus a new health-monitoring skill.

1. Phantom (previous) sessions on reconnect

When ResumeSessionAsync returns a different session ID (server assigned a new one), the old ID was not added to _closedSessionIds. The merge in SaveActiveSessionsToDisk would find both the old and new entries and rename the old one to (previous).

Fix: Mark old session ID as closed before updating to the new one.

2. ForceCompleteProcessingAsync uses ClearProcessingState

Last remaining site that manually cleared 15+ fields instead of calling the atomic ClearProcessingState helper. This was the code path used during reconnect bystander session cleanup.

3. Health monitoring skill

New .claude/skills/health-monitoring/SKILL.md for continuous PolyPilot session monitoring — covers single sessions, multi-agent, IDLE-DEFER, reconnect recovery audits, mobile/bridge/DevTunnel diagnostics.

Testing

• 3309/3309 tests pass
• Live validated: 2.5 hours of monitoring, 4 reconnects all handled correctly, zero new phantoms after fix

[PureWeen]

🔍 Multi-Model Code Review — PR #540

PR: fix: phantom sessions on reconnect + ForceComplete consolidation + health monitoring skill
Branch: fix/reliability-continued → main
Changes: +420 / -35 across 4 files (2 production, 1 test, 1 skill doc)
CI Status: ⚠️ No CI checks reported on this branch
Prior Reviews: None

---

Findings: None (consensus)

Three independent reviewers examined the full diff. Several single-reviewer findings were raised but none achieved consensus:

• ClearProcessingState not defined (1/3) — false positive. The method was introduced in PR fix: session reliability audit — eliminate stuck sessions, phantom (previous) entries, and DISPATCH-RECOVER false positives #531 (already on main). The reviewer checked the base branch without that context.

• preserveCarryOver: true lost (1/3) — intentional behavioral change. The old ForceComplete preserved stale deferred-idle fingerprints for no documented reason in a forced-recovery context. ClearProcessingState correctly clears them. Two reviewers explicitly noted this is correct. Discarded.

• AllowTurnStartRearm ordering (1/3) — this ClearProcessingState → AllowTurnStartRearm = false pattern is already used in 5 other call sites (AbortSessionAsync, watchdog timeout, watchdog crash, TriggerToolHealthRecovery, send error paths), all reviewed and approved in PR fix: session reliability audit — eliminate stuck sessions, phantom (previous) entries, and DISPATCH-RECOVER false positives #531. AllowTurnStartRearm is already false during active processing (set at SendPromptAsync turn start). Discarded.

• Test hollowed out (1/3) — the INV-1 field checks for ClearProcessingState's body already exist in ChatExperienceSafetyTests.cs (added in PR fix: session reliability audit — eliminate stuck sessions, phantom (previous) entries, and DISPATCH-RECOVER false positives #531). The ForceComplete test correctly now checks for delegation to ClearProcessingState rather than duplicating field-level assertions. Discarded.

---

✅ Verified Correct

• Reconnect phantom fix — _closedSessionIds[state.Info.SessionId] = 0 correctly added before state.Info.SessionId = actualId. No null check needed (both IDs validated by surrounding guard). _closedSessionIds is ConcurrentDictionary (thread-safe). FlushSaveActiveSessionsToDisk() called after both assignments.
• ForceCompleteProcessingAsync consolidation — response captured before ClearProcessingState clears buffers. API time accumulated manually before accumulateApiTime: false. AllowTurnStartRearm = false set after (matching 5 other approved call sites). WatchdogCaseA/B resets correctly left outside ClearProcessingState (force-complete-specific, not general processing state).
• Field completeness — ClearProcessingState additionally clears 5 fields the old code missed (ToolHealthStaleChecks, EventCountThisTurn, TurnEndReceivedAtTicks, IsReconnectedSend, LastUpdatedAt). Net improvement.
• Tests — structural tests correctly updated to verify delegation to ClearProcessingState instead of individual fields.
• Health monitoring skill — comprehensive 395-line skill doc; documentation-only, no production code impact.

---

🏁 Recommendation: ✅ Approve

Clean, focused reliability fix. The reconnect phantom session fix closes a real gap. ForceComplete consolidation follows the established ClearProcessingState pattern from PR #531. No consensus issues found.