Companion to mngr#2408: system_interface lifecycle state from the full observe stream

[gnguralnick]

Companion to mngr#2408 (the two land/merge separately).

Summary

Switch the system_interface agent manager from mngr observe --discovery-only (a state-less metadata stream that pinned every agent to a hardcoded RUNNING) to mngr observe --stream-events, whose AGENT_STATE / AGENTS_FULL_STATE / AGENT_REMOVED events carry each agent's real probed lifecycle state.

• Fold observe events into a per-id AgentDetails view: full-state rebuilds the set, agent-state upserts one, agent-removed drops one. AgentStateItem.state is now the real state; messaging identity comes from AgentDetails.host.
• A lifecycle-state change (e.g. RUNNING -> STOPPED when a process dies) re-gates the activity indicator through the unchanged is_agent_running gate, so a stopped agent drops its "Thinking..." indicator to idle.
• Only the source of the state changed; the liveness-dot / activity-indicator UI is untouched. _initial_discover is kept (it also seeds the assist-auto-open baseline and already carries real state).

Vendor

• Branch merged up to date with origin/main; the merge's only conflict was vendor/mngr, resolved by a coherent re-vendor of the whole tree from mngr bdec83414 (which contains the release36 SHA c55d2b225 that main vendored, plus the mngr#2408 change). Verified vendor-match: vendor/mngr equals mngr bdec83414 file-for-file (modulo the .minds/ files FCT's ignore strips).
• The system_interface fold change is isolated in its own commit for review; the vendor refresh + the merge are separate commits.

Testing

• Full apps/system_interface suite against the merged tree (main's new system_interface code + the re-vendored mngr): 557 passed (-m 'not tmux and not modal and not docker and not docker_sdk and not acceptance and not release' --no-cov). Two failures are environmental, not from this change: test_start_observe_spawns_long_lived_subprocess (mngr's is_allowed_in_pytest config guard aborts the real subprocess locally — flag-independent, passes in CI) and test_no_type_errors (local ty can't resolve modal/minds/latchkey; 0 errors in touched files).
• New/rewritten fold tests cover: AGENT_STATE STOPPED flips state + re-gates activity to IDLE; AGENTS_FULL_STATE rebuild; AGENT_REMOVED removal side-effects; location matching from AgentDetails.host; assist auto-open. ty clean on agent_manager.py; ruff clean.