feat(tasks): reuse existing sessions in new groups

[jerryccan]

Summary

Extend newgroup so an idle, unbound App Server Session can be reused in a new Feishu group without creating or forking a Session.

• Feishu: /newgroup [title] --session <session-id> [--agent <name>]
• CLI: agentbot task newgroup [task] [title] --session <session-id> [--agent <standard-name>]
• Accept raw Session IDs and codex://threads/<id> references.
• Preserve the existing new-task behavior when --session is omitted.
• Use --agent only to disambiguate the same Session ID across App Server Agents.

Behavior

Session state	Result
Idle and unbound	Create a group and atomically bind the exact existing Session.
Found on multiple Agents	Reject until --agent selects the owner.
Already bound or archived	Reject without changing the existing binding.
Active	Reject to avoid taking over in-flight work.
Identity or availability changes during group creation	Abort the binding and attempt to dissolve the new group once.

Session attachment is serialized per Agent and Session within a Worker, then enforced across processes by the existing SQLite uniqueness constraint. The remote Session is checked again after group creation before the local binding is committed. If the immediate group cleanup fails, the existing audit store marks the group fail-closed so incoming messages cannot silently create a different task. No persistent cleanup worker, retry loop, or new state table is introduced.

Session mode cannot be combined with --dir or --nodir. Mixed CLI/Worker versions fail before group creation instead of falling back to fresh-task behavior.

Validation

• npm run verify — unit/integration: 1074 passed, 3 skipped; process: 9 passed; render: 7 passed
• npm run build
• npm run package:smoke
• git diff --check origin/master...HEAD

@_user_1 please review.