chore(agent): commit the machine tier and the doctrine text the checkout was already running

Landing blocker for #61 and #63: the deployment checkout is 12 commits behind and
`merge --ff-only` refuses while tracked paths are dirty. Discard is not available here
(workspace policy: no `checkout`/`stash` on shared state), so the dirt is committed.

- `agent/APPEND_SYSTEM.md`: the point-of-use / one-work-log / PR-link doctrine, already
  byte-identical to `origin/main` (`cmp` against the trunk blob: no difference). No new
  prose — the commit exists so the trunk merge can proceed.
- `agent/config.yml` and `agent/managed-skills/**`: the machine-live tier the heartbeat
  owns (AGENTS.md § Live-sync tiers).

`ALLOW_SECRETS=1` was used, deliberately and after auditing the staged diff by hand: the
only credential-shaped strings in it are two copies of `AKIA0000000000000EXAMPLE` — the
inert all-zero AWS example id — and they appear inside the two skill pages that teach
that exact placeholder shape. This checkout is still running the pre-#61 hook; the merged
gate (value-scoped placeholder filter, `71601f7`) exempts this value, so the refusal
dies with this landing. No `--no-verify` was used: the gate ran and its verdict is
recorded here.

Refs #61

agent/managed-skills/grep-secret-gate-hook-traps/SKILL.md

@@ -0,0 +1,51 @@
+---
+name: grep-secret-gate-hook-traps
+description: "Write a zero-dependency grep-based pre-commit secret gate that fails closed: the GNU grep -- one-shot trap, POSIX ERE limits, bracket collating elements, and rc=2-as-success in command substitution."
+---
+
+# Fail-closed grep secret gate (pre-commit hook)
+
+Use when writing/auditing a bash pre-commit hook (or any CI tripwire) that scans
+staged content with `grep -E` for credential shapes — no gitleaks available.
+
+## The four traps (all personally earned; each silently PASSES bad content)
+
+1. **`--` is a ONE-SHOT end-of-options marker.** `grep -nE -- 'p1' -- 'p2' 'p3'`
+   takes the later `--` and patterns as FILE names → rc=2, errors on stderr.
+   Correct multi-pattern form: repeat `-e` for every pattern; never `--`.
+2. **`if hits=$(... | grep ...)` treats rc=2 as success.** Any non-zero-but-not-1
+   (broken pattern, unreadable file) enters the `then` branch with EMPTY output.
+   Never branch on the substitution's truthiness. Capture rc explicitly:
+   `hits=$(...); rc=$?` then: `rc>=2` → BLOCK (fail closed — a dead regex is
+   worse than none), `rc==1` → clean, `rc==0` → hits.
+3. **POSIX ERE (`grep -E`) has no `(?:...)` groups.** Use plain `(...)`.
+4. **`[:-]` inside a bracket expression starts a collating element** →
+   `grep: repetition-operator operand invalid` (rc=2, see trap 2). Put `-` FIRST
+   in every class: `[-A-Za-z0-9/+=_.:+]`.
+
+## Gate structure that works
+
+- Scan **staged content**, not the working tree: `git show ":$f"` over
+  `git diff --cached --name-only --diff-filter=ACM`.
+- Denylist doc/fixture paths (`README`, `docs?/`, `tests?/`, `*.example`) — else
+  the hook blocks its own patterns and every test fixture.
+- Report `BLOCKED <file>:<line>` only — never echo the matched secret.
+- Wire with tracked `hooks/pre-commit` + `git config core.hooksPath hooks`
+  (lives in the COMMON git dir config → one set in the main checkout covers
+  every worktree). Fresh clones are unprotected until it's set; document it.
+- Worktrees lacking `hooks/` while `core.hooksPath` is set: git dies on commit
+  (`cannot run ... No such file or directory`) — merge main in, don't unset.
+- Do NOT ship the hook in an npm plugin artifact (`files`): node_modules never
+  runs git; pin its ABSENCE in the pack test instead.
+
+## Test it red-first through the real seam (bats)
+
+`git init` a fixture repo, `cp` the real hook in, `git config core.hooksPath hooks`,
+then drive REAL `git commit` and assert:
+- credential-shaped `.env` (RFC-2606 domains / all-zero ids as inert fixtures) →
+  non-zero AND `git log` count unchanged (blocks, not warns);
+- PEM header line → blocked;
+- clean prose → passes;
+- the hook committing ITSELF → passes (trap 1/3/4 usually die here);
+- exit-code masking: `bats ... | tail` returns tail's rc — never `&& commit`
+  behind a piped suite, or you commit on red.