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).

scripts/type-equiv.manifest.json

@@ -10,6 +10,10 @@
     { "doc": "docs/core-data-structures/core.md", "symbol": "ToolSchema", "source": "packages/llm/llm/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "SessionEvent", "source": "packages/core/session/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "Agent", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "HookContext", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "PromptDecision", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "ContinuationDecision", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "SessionStartSource", "source": "packages/core/agent/src/types.ts" },
 
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "StreamChunk", "source": "packages/llm/llm/src/types.ts" },
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "TokenUsage", "source": "packages/llm/llm/src/types.ts" },
@@ -34,6 +38,8 @@
     { "doc": "docs/core-data-structures/tools.md", "symbol": "InferArgs", "source": "packages/core/tools/src/schema.ts" },
     { "doc": "docs/core-data-structures/tools.md", "symbol": "ToolExecution", "source": "packages/core/tools/src/index.ts" },
     { "doc": "docs/core-data-structures/tools.md", "symbol": "ToolExecutionResult", "source": "packages/core/tools/src/index.ts" },
+    { "doc": "docs/core-data-structures/tools.md", "symbol": "PreToolDecision", "source": "packages/core/tools/src/index.ts" },
+    { "doc": "docs/core-data-structures/tools.md", "symbol": "PostToolDecision", "source": "packages/core/tools/src/index.ts" },
 
     { "doc": "docs/core-data-structures/bash.md", "symbol": "BashExecRequest", "source": "packages/bash/bash/src/types.ts" },
     { "doc": "docs/core-data-structures/bash.md", "symbol": "BashExecSpec", "source": "packages/bash/bash/src/types.ts" },