Merge worktree-hooks-b-bash-seam into worktree-hooks-c-interception

Bring the interception-seams branch onto current master (via A→B). The
substantive reconciliation is master's compaction `agent/pre-step` serial seam
meeting C's interception seams:

- types.ts: keep BOTH master's `agent/pre-step` AND C's new interception events
  (`agent/prompt-submit`, `agent/session-start`, `agent/turn-continuation`→
  `ContinuationDecision`); drop the turn-mirror declarations (removed on A).
- loop.ts: the merged per-turn order is `turn/start` → per queued msg
  `agent/prompt-submit` (rewrite/inject/block) → (fully-blocked ⇒ zero-step
  `rejected`) → per step: drain steering → assemble system prompt →
  `agent/pre-step` (compaction, OUTSIDE the step) → `step/start` → single
  `deriveMessages()` → model → tools/pre-execute·dispatch·post-execute. No
  turn-mirror emits; `closeTurn()` is the A-simplified single-call form.
- Docs (architecture, core.md, agent/agent-loop READMEs, catalog) reconciled to
  show C's interception seams alongside `agent/pre-step`, no turn/step mirrors.
- rfc/README: dropped the stale `proposed/` compaction row (master moved that RFC
  to implemented/); kept C's new `pre-tool-input-rewrite` proposed row.
- interception.spec.ts: migrated its two `agent/turn-end` reason collectors to
  the `turn/end` session event, and ADDED a cross-test proving a
  `prompt-submit` rewrite + additionalContext is VISIBLE to an `agent/pre-step`
  listener on the same turn — pinning the merged seam ordering (compaction sees
  the post-prompt-submit surface, not stale history).

packages/support/invariants/src/index.ts

@@ -162,9 +162,6 @@ function checkEvent(trace: SessionTrace, event: SessionEvent): void {
       trace.surface.push(event.seq)
     } else {
       const { start, end } = se.surfaceOp
-      if (start > end) {
-        throw new InvariantError(`surface replace: start ${start} must be <= end ${end}`)
-      }
       const startIdx = trace.surface.indexOf(start)
       if (startIdx === -1) {
         throw new InvariantError(`surface replace: start seq ${start} is not on the surface`)

packages/support/invariants/tests/invariants.spec.ts

@@ -530,16 +530,17 @@ describe('surface invariants', () => {
     }).toThrow(/unknown seq 2/)
   })
 
-  it('rejects replace op with start > end', async () => {
+  it('rejects a replace whose start is positioned after its end on the surface', async () => {
     const { ctx } = await setup()
     const session = ctx.sessions.create()
     session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
     session.append('step/start', { turn: 1, step: 1 })
     session.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 2
-    // start > end is invalid (reversed order).
+    session.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 3
+    // Reversed range: start seq 3 is at a later surface position than end seq 2.
     expect(() => {
-      session.append('assistant/message', { turn: 1, step: 1, content: [] }, { surfaceOp: { op: 'replace', start: 2, end: 1 }, sourceEventSeqs: [2] })
-    }).toThrow(/must be <= end/)
+      session.append('assistant/message', { turn: 1, step: 1, content: [] }, { surfaceOp: { op: 'replace', start: 3, end: 2 }, sourceEventSeqs: [2, 3] })
+    }).toThrow(/is after end seq 2 .* on the surface/)
   })
 
   it('rejects a replace whose sourceEventSeqs omits a shadowed surface node', async () => {
@@ -608,6 +609,23 @@ describe('surface invariants', () => {
     }).toThrow(/is after end seq 4 .* on the surface/)
   })
 
+  it('accepts a replace whose start seq exceeds its end seq when the surface position order is valid', async () => {
+    const { ctx } = await setup()
+    const session = ctx.sessions.create()
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    session.append('step/start', { turn: 1, step: 1 })
+    session.append('user/message', { content: [{ type: 'text', text: 'a' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 2
+    session.append('user/message', { content: [{ type: 'text', text: 'b' }], source: { kind: 'user' } }, { surfaceOp: 'append' }) // seq 3
+    // Replace node 2 (position 0) with seq 4 — surface becomes [4, 3], so the
+    // head seq (4) is numerically GREATER than the tail seq (3): the surface is
+    // not seq-ordered. A replace spanning start=4 (pos 0) … end=3 (pos 1) is
+    // valid positionally and must be accepted even though start seq > end seq.
+    session.append('assistant/message', { turn: 1, step: 1, content: [{ type: 'text', text: 's' }] }, { surfaceOp: { op: 'replace', start: 2, end: 2 }, sourceEventSeqs: [2] }) // seq 4
+    expect(() => {
+      session.append('assistant/message', { turn: 1, step: 1, content: [] }, { surfaceOp: { op: 'replace', start: 4, end: 3 }, sourceEventSeqs: [4, 3] }) // seq 5
+    }).not.toThrow()
+  })
+
   it('rejects a replace that omits sourceEventSeqs entirely', async () => {
     const { ctx } = await setup()
     const session = ctx.sessions.create()