fix(hooks-claude): build subagent payloads from base(), run SubagentStop in the child cwd, drop agentType

Address the D agentType removal + two #124 review findings on the CC bridge's
subagent points:

- **Payloads from base()**: `subagentStart/StopPayload` bypassed `base()`, so the
  SubagentStart/SubagentStop stdin payloads omitted the CC-promised `session_id`
  and `cwd`. Replaced both with a single `subagentPayload()` built from `base(child)`
  (the child's session_id/cwd when the child is available) + `agent_id` +
  `agent_type` (+ `stop_hook_active` on Stop).
- **SubagentStop runs in the child cwd**: the listener called `runPoint(..., {})`
  with no agent, so the hook ran in the executor/server cwd. It now looks the
  child up via `ctx.get('agents').get(info.id)` — still recoverable because
  `subagent/end` fires from the service's detached `.then` BEFORE the tool caller
  disposes the child — and passes `{ agent: child }`, matching SubagentStart.
  New regression: server cwd ≠ child cwd, a `pwd` SubagentStop hook proves it
  ran in the CHILD workspace (proven red by neutering the lookup).
- **agent_type is a constant**: `info.agentType` no longer exists (removed on the
  subagent branch); both points now report the `SUBAGENT_TYPE = "general-purpose"`
  constant (Claude Code's Task-tool default), so a hooks.json default/`*`/empty
  `agent_type` matcher fires. Updated the README matcher-subject note and the
  bridge/coverage tests (dropped their agentType emits).
- **e2e comment**: hooks.e2e.ts said `./hooks.json` loads from the session cwd;
  corrected to process-level (server launch cwd), with the hook itself running
  in the session cwd.

packages/hooks/hooks-claude/tests/bridge.spec.ts

@@ -289,8 +289,8 @@ describe('hooks-claude bridge — SubagentStart / SubagentStop (observe)', () =>
     // Drive the observe-only lifecycle events directly (no real child needed — the
     // bridge just listens). The agents registry is absent here, so SubagentStart's
     // child lookup yields undefined and it simply runs the hook.
-    ctx.emit('subagent/start', { provider: 'inproc', id: AgentId('child-1'), agentType: 'researcher' })
-    ctx.emit('subagent/end', { provider: 'inproc', id: AgentId('child-1'), agentType: 'researcher', stopReason: 'completed', lastAssistantMessage: [{ type: 'text', text: 'done' }] })
+    ctx.emit('subagent/start', { provider: 'inproc', id: AgentId('child-1') })
+    ctx.emit('subagent/end', { provider: 'inproc', id: AgentId('child-1'), stopReason: 'completed', lastAssistantMessage: [{ type: 'text', text: 'done' }] })
 
     // Both hooks run async (detached .then); poll for their marker files rather
     // than a fixed sleep that flakes under load.