feat(events): interception seams — the typed-Decision surface for hooks

Reshape the agent's interception surface so every seam returns a small, typed
Decision union, and the set covers the hook points a CC/Codex bridge (and a
native plugin) needs. "Native hooks" are not a package — a native hook is just a
cordis plugin on these canonical events; the bridges (a later PR) only translate
an external protocol onto the same surface.

dsh-agent:
- NEW agent/session-start(agent, source) emit (once before turn 1; SessionStartSource
  startup|resume|clear|compact) — a pure notification, seeds context via inject().
- NEW agent/prompt-submit waterfall → PromptDecision (allow, optionally rewriting the
  prompt or attaching additionalContext, or block).
- RESHAPE agent/turn-continuation boolean → ContinuationDecision ({action:'stop'} |
  {action:'continue', reason?}; a continue reason is recorded as next-step steering).
- New HookContext envelope (required source — inject() would mislabel a missing one).

dsh-tools: split the single tools/execute waterfall into tools/pre-execute
(PreToolDecision allow/deny/ask gate) and tools/post-execute (PostToolDecision
accept/block, optionally replacing content or attaching additionalContext). Core
dispatch sits between as plain code; the tool body keeps its inner try/catch so a
thrown tool still reaches post-execute as an isError. ToolExecutionResult gains
additionalContext (ferried to the loop's per-step buffer). Input rewrite is
deliberately NOT offered (a proposed RFC designs it consistently).

dsh-session: new `rejected` TurnEndReason — a turn whose whole prompt batch was
blocked by prompt-submit.

agent-loop firing points: session-start emitted at create (source threaded —
startup for create/fork, resume for resume()); prompt-submit per drained message
with the always-open-turn rule (a fully-blocked batch is a zero-step rejected
turn); the continuation reshape; post-tool additionalContext buffered and appended
after all tool/results (adjacency). ACP codec maps rejected→cancelled.

A worked native-plugin example (interception.spec.ts) proves all four seams compose
end-to-end through the real loop with NO hook/* events (those belong to the bridge
lib). All existing tools/execute + turn-continuation tests migrated. The
tool-subagent abort test now aborts after a microtask so it still exercises the
live onAbort bridge (execute() awaits pre-execute before the body runs).

RFCs: implemented/feature/2026-06-30-interception-seams.md (the reshape) +
proposed/feature/2026-06-30-pre-tool-input-rewrite.md (the deferred rewrite design).

packages/bash/tool-bash/tests/integration.spec.ts

@@ -123,7 +123,10 @@ describe('bash tool through the agent loop', () => {
       textResponse('Background task finished.'),
     ])
     // The second tool call needs the REAL task id from the first result;
-    // a tools/execute waterfall listener rewrites the scripted arguments.
+    // a tools/pre-execute listener rewrites the scripted arguments. (This uses
+    // the low-level capability to mutate `exec` before dispatch — the
+    // unadvertised mechanism behind a future first-class input-rewrite decision;
+    // here it is a test shim to thread the generated id, not a product feature.)
     let taskId = ''
 
     const ctx = await harness(adapter)
@@ -137,7 +140,7 @@ describe('bash tool through the agent loop', () => {
         if (match) taskId = match[1]!
       }
     })
-    ctx.on('tools/execute', async (exec, next) => {
+    ctx.on('tools/pre-execute', async (exec, next) => {
       if (exec.name === 'bash_output') {
         exec.arguments = { task_id: taskId }
       }