feat: provision Alpine LXC 136 running Atheos, wired to Gitea

Provisions a LAN-only browser code editor (Atheos) in an Alpine 3.23 LXC on the
scotty PVE node, with armistace/blog cloned and ready to edit. Built via the
AI-DLC workflow; the full process trail is in aidlc-docs/.

Deliverables
- scripts/provision.sh          rebuild the guest from scratch (re-runnable)
- scripts/update-atheos.sh      move Atheos to latest upstream and RE-PIN
- scripts/build-and-test.sh     captures evidence for every acceptance criterion
- files/atheos.conf             nginx vhost (socket path byte-matches the FPM pool)
- files/ssh-config              ~atheos/.ssh/config deploy-key alias
- RUNBOOK.md                    operational guide (START AT STEP 1)
- docs/security-notes.md        credential model + wider-web hardening TODOs
- docs/resiliency-notes.md      what a non-PoC deployment would need

Design decisions
- D1 no git identity is configured by the agent; the operator sets it. Git will
  refuse the first commit until then, which is deliberate (requirement FR-11).
- D2 Atheos pinned to commit f90cd18 (build 611) with a verified tarball sha256,
  plus update-atheos.sh as the deliberate escape hatch.
- D3 nginx :80, web root /var/www/atheos, php-fpm over a unix socket.
- D6 git auth is a per-repository DEPLOY KEY scoped to armistace/blog, not a
  personal access token (a PAT's write:repository scope reaches every repo the
  account can write).

Verified on the live guest (40 checks, 0 failures)
- AC-2  Atheos served over HTTP 200 by nginx+php-fpm; socket paths byte-match
- AC-3  20 posts + themes/cleanblog present; owned by the php-fpm user
- AC-4  all five writable paths writable as the service user
- AC-5  remote targets armistace/blog; deploy key authenticates over SSH
- AC-6  survival proven with a real container restart, marker byte-identical
- AC-8  no python/pelican/pip/make and no DB engine - scope boundary enforced
- FR-11 git identity is empty; deploy key mode 600 owned by the service user

Traps documented in the RUNBOOK (each cost real debugging time)
- The Gitea SSH user is `gitea`, NOT `git` (app.ini SSH_USER=gitea). With `git`,
  sshd answers "Invalid user git" and rejects before reading authorized_keys,
  which is indistinguishable from a bad key.
- The Gitea SSH endpoint is the LAN address (:22 is not forwarded publicly).
- `uptime` and `free` report the PVE HOST inside an LXC without lxcfs, so a
  container that just restarted can claim days of uptime.

docs/resiliency-notes.md

@@ -0,0 +1,55 @@
+# Resiliency Notes — atheos-blog-lxc
+
+**Status**: resiliency-baseline extension **opted out** (LAN-only PoC) per Andrew's standing preference.
+**Purpose**: record what a non-PoC deployment would need, so the trail survives.
+
+---
+
+## 1. What this guest is, from a resiliency standpoint
+
+A **single small container that can be destroyed and rebuilt** — that is the design, not a gap.
+`armistace/blog` is the durability anchor: the source of truth lives in **Gitea**, not on this guest.
+Losing the LXC loses only:
+- the Atheos install (reproducible: `scripts/provision.sh`)
+- the local clone (re-clonable)
+- **uncommitted edits** ← the only genuinely perishable data
+
+## 2. Current posture
+
+| Aspect | State | Note |
+|---|---|---|
+| Rootfs | 20 GB, thin-provisioned on `scotty-local-lvm` | thin pool had 771 GB free at design time |
+| `onboot` | enabled | survives a node restart |
+| Backups | **none** | acceptable: nothing unique is stored |
+| Snapshots | none taken (a `pre-*` snapshot is used during *upgrades* only) | |
+| Rebuild path | `scripts/provision.sh` | **this is the resiliency mechanism** |
+| Reboot survival | **verified**, not assumed | AC-6 — service returns and the clone persists |
+
+## 3. If this ever moves beyond a LAN PoC
+
+| # | Item | Touch-point |
+|---|---|---|
+| R-1 | **Scheduled snapshot** of the guest (`pct snapshot` / PBS) with retention | PVE / PBS |
+| R-2 | **Automated verification of the rebuild path** — run `provision.sh` into a throwaway VMID on a schedule; a backup you have never restored is a claim | CI or cron on the PVE node |
+| R-3 | **Alert on guest down** (`pct status` + HTTP probe) | observability stack |
+| R-4 | **Push-based durability for in-progress work**: an auto-commit/ahead-of-time branch so uncommitted edits stop being the failure mode | a small hook or Atheos-side job |
+| R-5 | **Disk alert at 80%** of the 20 GB rootfs (a full rootfs silently breaks PHP sessions) | observability stack |
+| R-6 | **Move the clone's durability to the repo**: rely on Gitea backup/restore rather than guest backups | Gitea |
+| R-7 | **Document RTO/RPO**: here RTO ≈ re-run `provision.sh` (minutes); RPO = last commit | RUNBOOK |
+| R-8 | **`scotty-local-lvm` thin-pool monitoring** — a thin pool that fills takes down *every* guest on it, not just this one | PVE node |
+
+## 4. Design choices that already help
+
+- **Nothing unique is stored on the guest** — the deliberately tiny 20 GB root is a *feature*: it
+  bounds what can be lost and keeps rebuild cheap.
+- **Provisioning is a script**, so rebuild is not a memory exercise (FR-9).
+- **The build pipeline is elsewhere** (Q1=A), so this guest is not on the critical path for publishing.
+- **OpenRC services are enabled at boot** (AC-6 verifies it), so a reboot is not a repair event.
+
+## 5. Accepted risks
+
+| Risk | Why accepted | Revisit trigger |
+|---|---|---|
+| No backups | no unique data; rebuild is scripted | if the guest starts holding anything unique |
+| Single guest on a single node | LAN PoC scope | on any production expectation → R-1/R-3 |
+| Uncommitted edits are perishable | normal for a git worktree; operator controls commit cadence | if it bites → R-4 |