Make wta-master the sole WT event subscriber

[DDKinger]

Problem

Each wta-helper currently starts its own wtcli listen subscription to the Terminal COM event broadcast. The master now owns authoritative agent_event routing, so helpers no longer forward agent CLI hooks; however, every helper still receives and parses every window-wide WT event to maintain synchronous pane-local state for:

• pane → agent-session binding (is_agent_pane, key_for_pane, origin_for_pane)
• shell-agent exit inference from OSC 133;A
• pane-local Autofix classification and target selection

With N tabs, one event is still delivered and parsed N times. Most cross-tab events are discarded only after they have crossed the COM/process boundary.

Goal

Make the master the only window-wide WT event subscriber. Helpers should receive only owner-scoped, typed state/events from master rather than subscribing independently to the global COM stream.

Proposed direction

1. Keep wta-master as the sole consumer of agent_event, connection_state, tab lifecycle, and other global WT events.
2. Have master publish typed pane/session binding deltas to the owning helper over the existing helper pipe.
3. Preserve the helper's synchronous local lookup model — update its local AgentSessionRegistry from those typed master notifications before pane-local input/UI decisions need the data.
4. Replace the high-volume raw vt_sequence dependency for shell-agent exit detection with either:
   • a narrow master-routed shell_prompt_started event emitted by Terminal for OSC 133;A, or
   • an owner-scoped notification from master after it filters the global VT stream.
5. Remove helper CliChannel::subscribe_events / wtcli listen processes once every helper consumer has a typed master-owned source.
6. Consider replacing the master's wtcli listen child process with an in-process COM ITerminalProtocol::Subscribe client so subscription readiness and lifetime are owned in one process.

Constraints

• Do not make helper UI/input wait on a synchronous round-trip to master; pane binding must remain locally queryable.
• Preserve per-tab and per-window identity. Never broadcast a pane mutation to unrelated helpers.
• Preserve shell-pane Gemini /exit detection when the pane remains alive and no SessionEnd hook arrives.
• Preserve graceful degradation: if the authoritative event subscriber is unavailable, chat, Autofix, history listing and resume remain operational; only real-time session status may be stale until reconnection.
• Do not regress hook failure isolation: agent CLI hooks must remain silent and exit 0 when Terminal is unavailable.

Acceptance criteria

• One WT event causes one global parse/routing operation in master, not one per helper.
• A helper receives only events/deltas relevant to its owner tab/window.
• No helper owns a wtcli listen child process.
• Shell-agent OSC 133;A exit detection, connection failures, pane close, Autofix targeting, tab drag/rename, and session focus/resume continue to pass their existing unit and ItE2E suites.
• A multi-tab ItE2E proves that an event in tab A does not cross into helpers for tabs B/C while tab A's local pane binding updates before its next decision.