Infrastructure pass: CI smoke tests, floating versions, chown sentinel, generate-config script

Main changes:

- Extract opencode.json generation from entrypoint-user.sh into a
  standalone Python script (rootfs/usr/local/lib/opencode-devbox/
  generate-config.py). Preserves the never-overwrite-existing-config
  guarantee. Cuts entrypoint-user.sh from 176 to 97 lines.

- Install MemPalace via 'uv tool install' into an isolated venv at
  /opt/uv-tools/mempalace/ with a /usr/local/bin/mempalace-mcp-server
  wrapper, replacing the 'pip install --break-system-packages' escape
  hatch. The wrapper is what generate-config.py references in the
  auto-generated opencode.json. Also fix 'mempalace init' in
  entrypoint-user.sh to use --yes so first-start initialization isn't
  interactive (this used to hang or print prompts into the user's
  terminal). Gated by INSTALL_MEMPALACE build arg (default true) so
  users who don't need AI memory can shave ~300 MB.

- Sentinel-file pattern in entrypoint.sh volume-ownership loop: write
  .devbox-owner after a successful chown -R, skip the recursive walk
  on subsequent starts when the sentinel matches FINAL_UID:FINAL_GID.
  Cuts multi-second startup costs to milliseconds on large volumes
  (nvim plugins, palace data). UID changes still trigger a full chown.

- Float all GitHub/Gitea-hosted binary versions: gosu, fzf, git-lfs,
  neovim, bat, eza, zoxide, uv, gitea-mcp now default to 'latest' and
  resolve the newest upstream release at build time via the /releases/
  latest redirect. Go (go.dev JSON feed) and oh-my-opencode-slim (npm
  @latest) likewise. Intentional pins still in place: OPENCODE_VERSION,
  NODE_VERSION=22, DEBIAN_VERSION=trixie-slim. Each *_VERSION ARG
  accepts an explicit value to lock a specific version when needed.

- New scripts/smoke-test.sh verifies binary presence, opencode startup,
  entrypoint user drop, generate-config idempotency, bun's presence-
  per-variant, and image size against thresholds (2500 MB base, 3000
  MB OMOS). Prints resolved component versions as its first step so
  CI logs always record what got baked into a given image.

- New .gitea/workflows/validate.yml runs on push to main and PRs:
  single-arch amd64 build, smoke test, DOCKER_HUB.md sync check. Tag-
  triggered docker-publish.yml now smoke-tests each variant on amd64
  before the full multi-arch push.

- scripts/generate-dockerhub-md.py auto-generates DOCKER_HUB.md from
  README.md using explicit SECTION_RULES. --check mode fails CI when
  the committed file is out of sync. Enforces the 25 kB Docker Hub
  limit. Adding a new README section forces an explicit keep/drop/
  replace decision.

- Remove dead INSTALL_PYTHON build arg (was a no-op since mempalace
  added python3 unconditionally).

README.md

@@ -327,18 +327,24 @@ docker compose run --rm --build devbox
 
 ### Build Args
 
-Enable optional language runtimes or pin a specific opencode version:
+Enable optional language runtimes, pin a specific opencode version, or lock any of the tooling components:
 
 ```bash
 docker compose build --build-arg INSTALL_GO=true
 docker compose build --build-arg OPENCODE_VERSION=1.5.0
+docker compose build --build-arg NVIM_VERSION=0.12.1   # pin to a specific version
 ```
 
 | Arg | Default | Description |
 |---|---|---|
-| `INSTALL_GO` | `false` | Go toolchain |
+| `INSTALL_GO` | `false` | Go toolchain (resolves latest stable from go.dev when `GO_VERSION=latest`) |
+| `INSTALL_MEMPALACE` | `true` | [MemPalace](https://github.com/MemPalace/mempalace) local AI memory system (~300 MB — disable to shrink image if you don't need MCP memory) |
 | `INSTALL_OMOS` | `false` | [oh-my-opencode-slim](https://github.com/alvinunreal/oh-my-opencode-slim) multi-agent orchestration (installs Bun and plugin) |
-| `OMOS_VERSION` | `latest` | Pin a specific oh-my-opencode-slim version |
+| `OPENCODE_VERSION` | *(pinned per release)* | opencode npm version. Drives the image tag and is intentionally not floated. |
+| `NODE_VERSION` | `22` | Node.js major version. Pinned to protect against upstream breaking changes across majors. |
+| `GOSU_VERSION`, `FZF_VERSION`, `GIT_LFS_VERSION`, `NVIM_VERSION`, `BAT_VERSION`, `EZA_VERSION`, `ZOXIDE_VERSION`, `UV_VERSION`, `GITEA_MCP_VERSION`, `GO_VERSION`, `OMOS_VERSION` | `latest` | All GitHub/Gitea/go.dev-hosted binaries resolve to the newest upstream release at build time. Override with a specific version to pin. Resolved versions are logged in CI output. |
+
+> **Reproducibility note:** With `latest` defaults, two builds of the same `v{opencode}` tag may embed different tool versions if upstream releases have happened in between. This is intentional — it means every rebuild picks up upstream CVE fixes automatically. If you need a bit-for-bit reproducible build, pass explicit `*_VERSION` args. The CI smoke test logs the resolved versions for every release build.
 
 ## oh-my-opencode-slim (Multi-Agent Orchestration)
 
@@ -447,6 +453,8 @@ SSO sessions typically last 8–12 hours before requiring re-authentication. Sin
 
 The image includes [MemPalace](https://github.com/MemPalace/mempalace), a local-first AI memory system that stores conversation history verbatim and retrieves it via semantic search. Nothing leaves your machine.
 
+> MemPalace adds ~300 MB to the image (chromadb, embedding model deps). If you don't use it, rebuild with `--build-arg INSTALL_MEMPALACE=false` to shrink the image.
+
 ### Enabling persistence
 
 Uncomment the palace volume in `docker-compose.yml`:
@@ -466,12 +474,14 @@ Add mempalace as an MCP server in your `opencode.json` (inside `~/.config/openco
   "mcp": {
     "mempalace": {
       "type": "local",
-      "command": ["python3", "-m", "mempalace.mcp_server"]
+      "command": ["mempalace-mcp-server"]
     }
   }
 }
 ```
 
+> The image installs mempalace into an isolated `uv tool` venv at `/opt/uv-tools/mempalace`. The `mempalace-mcp-server` wrapper on `PATH` exec's the venv's Python with the `mempalace.mcp_server` module — you don't need to know about the venv to use it.
+
 This gives opencode access to 29 MCP tools for searching memory, querying the knowledge graph, managing wings/rooms/drawers, and agent diaries.
 
 ### Basic usage