Archiving a thread should archive its native provider session

[alexaka1]

Problem

Archiving a thread in T3 Code hides it in T3 and detaches its provider session, but no provider is told. The thread stays active in Codex, OpenCode, and every other harness that has its own session list, so anyone who uses both ends up archiving twice.

It is worse in reverse: if I archive the thread in Codex myself and T3 later resumes it, the Codex adapter unarchives it again (CodexAdapterV2.ts, the thread/unarchive retry on "session … is archived").

Current behavior (main @ 6524a8d)

• thread.archive in apps/server/src/orchestration-v2/Orchestrator.ts cancels queued work, emits provider-session.detach (with revokeMcpCredential), and queues terminal.cleanup. Nothing reaches the provider.
• No adapter archives or deletes its native session on archive. The only native session mutation is the manual ACP delete under Settings → Native sessions, which is unrelated to archive and refuses while a T3 thread references the session.

What each provider supports natively

Provider	Native archive	Native delete	Source
Codex	thread/archive, thread/unarchive	—	packages/effect-codex-app-server/src/_generated/meta.gen.ts
OpenCode 1.x	session.update({ time: { archived } })	session.delete	@opencode-ai/sdk 1.19 dist/v2/gen/sdk.gen.d.ts
OpenCode 2	time.archived is on the session, but SessionUpdateInput in @opencode/client 2.0.23 cannot set it	—	@opencode/client 2.0.23
Cursor	Agent.archive / Agent.unarchive, documented as cloud-only; T3 runs local agents	cloud-only	@cursor/sdk 1.0.35 stubs.d.ts
Claude	—	deleteSession() (removes the transcript)	@anthropic-ai/claude-agent-sdk 0.3.x sdk.d.ts
ACP agents (incl. Grok)	—	session/delete, when advertised	packages/effect-acp
Antigravity, Muse, Pi	—	—	adapters and SDK types

Proposal

1. Give provider adapters optional archiveNativeSession / unarchiveNativeSession capabilities, implemented where the provider has a native archive: Codex and OpenCode 1.x today, OpenCode 2 and Cursor local agents once their APIs allow it.
2. thread.archive archives each of the thread's native sessions (including subagent sessions once feat(server): archive and unarchive cascade to subagent threads #18668 lands), and thread.unarchive reverses it.
3. Providers without a native archive are skipped.
4. A native archive failure is logged and never fails the T3 archive.

Deletion (Claude, ACP) is deliberately left out. T3 resumes and forks from the native session, so deleting it on archive would make unarchive lossy. If there is appetite for it, it belongs in thread deletion rather than archive.