feat: add writer-branch workflow macros

Adds the branch-based publishing model: the editor writes to a long-lived
`atheos-writer` branch, and merging into `master` stays in Gitea.

Six macros installed into Atheos's macro store (a copy is kept in
files/macros.db.php so a rebuild reproduces them), invoked by right-clicking
the `blog` folder:

  1. Start/Join Writer Branch            idempotent create-or-switch + push -u
  2. Commit + Push Writer Branch         identity guard -> add -A -> commit (only
                                         if something is staged) -> push
  3. DESTRUCTIVE: Reset Writer to Master fetch -> reset --hard origin/master ->
                                         push --force-with-lease
  4. Status + Recent Log                 read-only
  5. Refresh Master (ff-only)            pull master down, return to writer
  6. Merge Master Into Writer Branch     when master has moved ahead

Atheos has no terminal and codegit exposes no branch-switching, so macros are
the only way to run arbitrary git from the UI. Verified: codegit's `checkout`
action takes a FILE argument - it is per-file revert, not branch switching.

Identity handling: the commit macros deliberately do NOT hardcode an identity.
php-fpm runs with HOME=/home/atheos (verified), the same global gitconfig the
operator edits over SSH, so hardcoding `git -c user.name=...` would OVERRIDE the
operator's own identity. Instead macro 2 guards on an identity existing and fails
with an actionable message - which is what design decision D1 intends.

Verified by exercising each command through the web server as the php-fpm user:
  - macro 1, 4, 5 return rc=0 with correct output
  - macro 2's identity guard fires with the exact RUNBOOK STEP 1 message
  - macro 3 was tested with real divergence: committed on the writer branch,
    reset --hard to master, force-pushed; origin/atheos-writer then matched
    origin/master exactly (0 ahead, 0 behind)

Security note recorded in the docs and README: this model only holds if branch
protection is enabled on armistace/blog's master. It is currently NOT enabled,
so the deploy key could force-push master directly - the operator needs to turn
it on (repo-owner action).

Also documents two hazards found while testing:
  - macro 2's `git add -A` stages everything, and a subsequent `reset --hard`
    deletes any untracked file that was staged. Keep scratch files outside
    workspace/blog/.
  - recovery procedure for a file lost that way (git fsck --lost-found ->
    ls-tree -> cat-file), which was used to restore a file during testing.

README.md

@@ -55,10 +55,12 @@ posts and commit them to Gitea — without VS Code, without a cloud IDE, and wit
 ├── RUNBOOK.md                    ← operational guide; START AT STEP 1
 ├── docs/
 │   ├── security-notes.md         credential model + hardening TODOs if ever exposed
-│   └── resiliency-notes.md       what a non-PoC deployment would need
+│   ├── resiliency-notes.md       what a non-PoC deployment would need
+│   └── writer-branch-workflow.md how writing/publishing works from the editor
 ├── files/
 │   ├── atheos.conf               nginx vhost (install to /etc/nginx/http.d/atheos.conf)
-│   └── ssh-config                ~atheos/.ssh/config (the gitea-blog alias)
+│   ├── ssh-config                ~atheos/.ssh/config (the gitea-blog alias)
+│   └── macros.db.php             installed copy of the Atheos writer macros
 ├── scripts/
 │   ├── provision.sh              rebuild the whole guest from scratch
 │   └── update-atheos.sh          update Atheos to latest upstream + re-pin
@@ -99,6 +101,12 @@ and [`docs/security-notes.md`](docs/security-notes.md).
 
 ## Security at a glance
 
+- **Writing happens on a branch, never on `master`.** The editor commits to a long-lived
+  `atheos-writer` branch and you merge via PR in Gitea — see
+  [`docs/writer-branch-workflow.md`](docs/writer-branch-workflow.md).
+  ⚠️ This only holds if **branch protection is enabled on `armistace/blog`'s `master`** — without it
+  the deploy key could force-push master directly. Confirm with the `branch_protections` API call in
+  that document.
 - Git auth: **deploy key scoped to `armistace/blog`**, private key mode `600`, owned by the php-fpm user.
 - The SSH alias uses `IdentitiesOnly yes`, so no other key is ever offered.
 - The web app runs as a **dedicated unprivileged user** (`atheos`), which also owns the workspace.