fix(hooks): run hooks in the session cwd; honest process-level config + best-effort session-start; surface systemMessage drop

Address review on the bridges:

- Hook cwd (blocking): the bridges never passed a workdir to runHook, so hooks
  ran in the executor default (the ACP server launch dir), not the session
  cwd — a hook doing `pwd`/relative reads/marker writes operated in the wrong
  tree. Both bridges now thread the agent's session `header.cwd` (the
  session/new.cwd) as the hook workdir for agent-scoped points. Regression per
  bridge: server cwd ≠ session cwd, a `pwd` hook proves it ran in the session
  workspace (proven red without the workdir).
- Example config honesty (blocking): `configPath: ./hooks.json` is read ONCE at
  load against the PROCESS cwd, not per-session — the comment/README now say so
  explicitly (a project-local per-session hooks.json is not discovered;
  TODO(per-session-hook-config)). The hooks-run-in-session-cwd fix above is the
  distinct, separately-documented half.
- Session-start timing (blocking): agent/session-start is a synchronous emit and
  the hook runs on a detached .then, so injected context is BEST-EFFORT — not
  guaranteed before the first request. Downgrade the contract in code comments +
  README + RFC (TODO(session-start-gating)) rather than implying "first request
  sees it", and add a no-wait regression that asserts the safe properties
  without pre-waiting for the inject.
- systemMessage (non-blocking): the merge collects merged.systemMessages but no
  bridge surfaced it. Warn per hook (like updatedInput) and document it as
  deferred in both READMEs + the RFC; tests assert the warn + non-surfacing.

docs/rfc/implemented/feature/2026-06-30-hook-bridges.md

@@ -39,13 +39,18 @@ Each bridge maps the neutral `MergedHookOutcome` from the shared lib onto the se
 
 The config is parsed ONCE at load; a read/parse failure logs and registers nothing rather than crashing boot (a typo'd path must not take the agent down). Only `type: 'command'` hooks run — a `prompt`/`agent`/HTTP hook (CC) or an `async: true` / non-command hook (Codex) is parsed-and-skipped with a warning. The emit-listener paths (`session-start`, `subagent/start`) run detached, with their `inject` contained in a `.catch` that logs (a throwing inject must not break session boot or the loop).
 
+### Where hooks run, and where their config comes from
+
+Two different cwds, kept distinct on purpose. The hooks **themselves** run in the agent's **session workspace**: for the agent-scoped points the bridge threads the session's `cwd` (`session/new.cwd`, on the session header) to `runHook` as the process working directory, so a hook's `pwd` / relative-file read / marker write operates in the user's project tree, not the server's launch directory. The **config path**, by contrast, is **process-level**: `configPath` is resolved and parsed once at load against the process launch cwd, so a single `hooks.json` applies to the whole process — there is no per-session config discovery that reads a project-local `hooks.json` from each `session/new.cwd` (`TODO(per-session-hook-config)`). This is an honest limitation of the current cut: the example `cordis.yml` documents that its `./hooks.json` is process-level, not per-project.
+
 ## Deferred (faithful-but-degraded)
 
 - **Tool-input rewrite.** A CC/Codex `updatedInput` is logged + warned, not honored — input rewrite is a deferred consistency-design problem ([the pre-tool-input-rewrite RFC](../../proposed/feature/2026-06-30-pre-tool-input-rewrite.md)), because the pre-execution args are read by `tool/call` audit + `assistant/message` history + ACP/tool-bash presentation, so an honest rewrite is a design unit, not a field.
 - **Stop loop-guard** (`TODO(stop-loop-guard)`). CC/Codex break an infinite force-continue with `stop_hook_active` (true once a Stop hook fired this run) plus a max-consecutive cap; both are deferred. Today `stop_hook_active` is always `false`, so a Stop hook that unconditionally blocks would force-continue every step — a hook author must self-limit until the guard lands.
 - **Permission `ask`** degrades to `deny` at the `tools/pre-execute` seam (`FIXME(permissions)` in the interception-seams RFC) — there is no interactive permission prompt yet.
 - **Hook `continue:false` (hard halt).** A hook can ask to halt the whole run (CC/Codex `continue:false`); the shared merge folds it into `MergedHookOutcome.stop`/`stopReason`, but no bridge acts on it (`TODO(hook-continue-false)`) — the interception seams have no "hard-halt the agent" primitive yet (a Decision blocks/steers a single point, not the run). Deferred with the loop-guard work; the halt request is recorded in the `hook/result` log, and the hook keeps its per-point effect (decision/context) meanwhile.
-- **Config discovery.** The path is explicit in `cordis.yml`; the full multi-layer CC/Codex precedence walk and the trust/hash model are not reimplemented (`TODO`).
+- **Config discovery.** The path is explicit in `cordis.yml` and process-level (see above); the full multi-layer CC/Codex precedence walk, per-session project-local discovery, and the trust/hash model are not reimplemented (`TODO(per-session-hook-config)`).
+- **Session-start / subagent-start context is best-effort, not gated (`TODO(session-start-gating)`).** `agent/session-start` is a synchronous emit and the bridge runs its hook on a detached `.then`, so the injected `additionalContext` is not guaranteed to land before the first turn reaches the model — a slow hook can miss the first request (the context then arrives as a later injection). `subagent/start` is sharper: an in-process provider may have already queued the child's prompt before the listener runs, and a short-lived child can finish before the detached inject fires. Making startup context a gated/awaited primitive is a loop-level change deferred to the interception seams; today the contract is "injected as soon as the hook resolves", not "before the first request". The bridge tests do NOT wait on the injection where they assert the guaranteed-timing behavior, so they document the real (best-effort) timing rather than masking it.
 
 ### Multiple hooks on one point run serially, not concurrently