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/gitea-credential-store-url-format/SKILL.md

@@ -0,0 +1,42 @@
+---
+name: gitea-credential-store-url-format
+description: "Auth to the Dankclaw Gitea API without the empty-header trap: ~/.config/secrets/gitea is a git-credential-store URL file, so awk '/^password=/' returns empty; resolve the token via git credential fill, and Gitea writes need Authorization: token (reads pass anonymously on a public repo)."
+---
+
+# Gitea auth on Dankclaw: the URL-format credential store
+
+`~/.config/secrets/gitea` is `git config credential.helper = store --file ~/.config/secrets/gitea`, i.e. a **git-credential-store URL file**:
+
+```
+http://dank:<40hex-token>@192.168.0.143:3000
+```
+
+It does NOT contain `username=`/`password=` lines. So:
+
+```bash
+# WRONG — silently returns empty string
+p=$(awk -F= '/^password=/{print substr($0,10)}' ~/.config/secrets/gitea)
+```
+
+An empty `$p` makes every curl send `Authorization: token ` (blank) → Gitea replies `{"message":"token is required"}` on **writes**, while **reads still return 200** (the repo is public-readable, so a green `GET /labels` or `/issues` proves nothing about auth). Symptom: PR-create returns `token is required` even though a `GET` just "worked".
+
+## Resolve the real token via git
+
+```bash
+p=$(printf 'protocol=http\nhost=192.168.0.143:3000\n\n' \
+     | git credential fill 2>/dev/null \
+     | awk -F= '/^password=/{print substr($0,10)}')   # 40-hex token
+```
+
+## Write auth = token scheme, via a 600-perm -K file
+
+```bash
+cfg=$(mktemp); chmod 600 "$cfg"
+printf 'header = "Authorization: token %s"\n' "$p" > "$cfg"
+curl -s -K "$cfg" http://192.168.0.143:3000/api/v1/user   # confirms identity
+curl -s -K "$cfg" -X POST -H 'Content-Type: application/json' --data @body.json "$API/pulls"
+```
+
+- Never pass the token with `-H` — it lands in `/proc/<pid>/cmdline`, world-readable.
+- Confirm identity with `GET /user` before trusting a write path; a `token is required` there means the token never authenticated.
+- Merges are a separate shape (form-encoded `do=merge`, auth-only cfg, NO `Content-Type` header) — see the gitea-merge-form-encoded skill.