feat: plan agent v2, global chat mode, and user-mode v2 flag sync

- Add runtime/plan_agent_v2 package and shims; gateway/direct_loop/WS wiring

- Admin chat: interaction mode and specialist only in user menu; session API stores memory_mode and execution_mode only

- POST /admin/api/chat/user-mode mirrors plan_agent_version to AIA_EXPERT_PLAN_AGENT_V2_ENABLED (v2 to 1, v1 to 0)

- Composer cleanup (hidden mode select, no reasoning toggle in meta bar); tests and _local/system.env.example

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/plan_agent_v2_gateway_cutover_patch.md

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+# Plan Agent V2 Gateway Cutover Draft
+
+## Purpose
+- Provide a minimal, reviewable gateway cutover sketch without changing production routing yet.
+- Keep existing `runtime/gateway.py` behavior unchanged until explicit cutover approval.
+
+## Draft Helper
+- New module:
+  - `runtime/plan_agent_v2_gateway_cutover.py`
+- Entrypoint:
+  - `maybe_handle_expert_turn_v2_draft(...)`
+
+## Draft Behavior
+- If v2 shadow is not selected:
+  - returns `handled=False`, gateway should continue legacy flow.
+- If decision is `enter_plan` or `stay_plan`:
+  - returns `handled=True` with an `OclawGatewayResult` built from v2 shadow compatibility mapper.
+- If decision is `run_agent`:
+  - returns `handled=False` and provides `system_prompt_override`.
+  - gateway would continue legacy execution path but with injected approved-plan context.
+
+## Why This Is Safe
+- No import or call-site changes in `runtime/gateway.py` yet.
+- Feature remains effectively dormant unless future cutover patch wires this helper.
+- Existing tests continue to validate legacy and shadow independently.
+
+## Future Minimal Cutover (single commit)
+- In `OclawGateway.handle_turn(...)` expert path, add one early branch:
+  1) call `maybe_handle_expert_turn_v2_draft(...)`
+  2) if `handled=True`, return result immediately
+  3) else continue existing flow; if `system_prompt_override` exists, use it as specialist system prompt
+
+## Rollback
+- Revert only the gateway wiring commit.
+- Keep shadow modules and tests as dormant assets.
+