feat(group-chat): add mixed-runtime participants

[wtj-0527]

Summary

This Draft PR adds mixed-runtime Hermes Studio Group Chat participants while preserving participant identity, runtime isolation, fixed-chain request provenance, and fail-closed lifecycle behavior.

Runtime and identity

• Keeps Hermes, Codex, and Claude participants bound to their own runtime/profile/session identity.
• Prevents ordinary Coding Agent chat sessions and Group Chat participant sessions from reusing one another's runner/native session identity.
• Preserves the original durable depth-0 user request as trusted chain_request data while keeping predecessor output as separate untrusted context.

Lifecycle hardening

• Adds durable runtime mutation fences with token, Room, actor scope, kind, and expiry checks.
• Adds heartbeat lease renewal and fail-closed participant deletion, Room deletion, and context clearing.
• Uses reference-counted local Room admission pauses; each lifecycle releases only its own lease.
• Binds participant fence consumption to token + roomId + actorId + kind='participant' inside the destructive transaction and rolls back when exactly one fence is not consumed.
• Keeps heartbeat/fence cleanup in nested finally blocks even when runtime finalizers fail.
• Restricts cleanup to ownership-proven Group Chat Coding Agent sessions and their owned evidence.

Exact candidate identity

• PR head: 76efd3e5a0db496b07600b3b11b4f1b14daf4e11
• Git tree: 8455627d974f92ddc1e047afb33a94572b895be4
• Changed paths: 23
• Immutable-review diff SHA256: bf6af797710d65d98e4d41732d60ffcfb744dbd70770d33da5552838325e5908
• Independent immutable-tree review: PASS

Local verification

All commands below were run against the exact candidate bytes before push:

• Directly related lifecycle tests: 3 files / 118 tests passed
• All changed server test files: 14 files / 289 tests passed
• Server TypeScript (tsc --noEmit): PASS
• Client TypeScript (vue-tsc -b): PASS
• Harness contract (npm run harness:check): PASS
• Production build (npm run build): PASS
• Immutable disposable snapshot build: PASS
• git diff --check: PASS

The broad repository suite has a known non-green baseline and is not represented here as green. The focused candidate gates above are the authoritative local evidence.

Current delivery state

• PR remains Draft.
• No merge or auto-merge is enabled.
• GitHub CI must be evaluated only for exact head 76efd3e5a0db496b07600b3b11b4f1b14daf4e11; checks from earlier heads are superseded.
• No new image or LPK is claimed by this revision yet. A unique test package will be produced only after exact-head CI succeeds.
• No installed Hermes Studio instance was modified, restarted, replaced, or deployed during this work.