feat(policy): add lane reconciliation events and policy support

Add terminal lane states for when a lane discovers its work is already
landed in main, superseded by another lane, or has an empty diff:

LaneEventName:
- lane.reconciled — branch already merged, no action needed
- lane.merged — work successfully merged
- lane.superseded — work replaced by another lane/commit
- lane.closed — lane manually closed

PolicyAction::Reconcile with ReconcileReason enum:
- AlreadyMerged — branch tip already in main
- Superseded — another lane landed the same work
- EmptyDiff — PR would be empty
- ManualClose — operator closed the lane

PolicyCondition::LaneReconciled — matches lanes that reached a
no-action-required terminal state.

LaneContext::reconciled() constructor for lanes that discovered
they have nothing to do.

This closes the gap where lanes like 9404-9410 could discover
'nothing to do' but had no typed terminal state to express it.
The policy engine can now auto-closeout reconciled lanes instead
of leaving them in limbo.

Addresses ROADMAP P1.3 (lane-completion emitter) groundwork.

Tests: 4 new tests covering reconcile rule firing, context defaults,
non-reconciled lanes not triggering reconcile rules, and reason
variant distinctness. Full workspace suite: 643 pass, 0 fail.

rust/crates/runtime/src/lib.rs

@@ -100,7 +100,7 @@ pub use plugin_lifecycle::{
 };
 pub use policy_engine::{
     evaluate, DiffScope, GreenLevel, LaneBlocker, LaneContext, PolicyAction, PolicyCondition,
-    PolicyEngine, PolicyRule, ReviewStatus,
+    PolicyEngine, PolicyRule, ReconcileReason, ReviewStatus,
 };
 pub use prompt::{
     load_system_prompt, prepend_bullets, ContextFile, ProjectContext, PromptBuildError,