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).

CHANGELOG.md

@@ -8,7 +8,20 @@ Tags follow `v{opencode_version}[letter]` — bare tag for the first build on a
 
 ## Unreleased
 
+**Infrastructure and maintainability pass.**
+
 - **Cleanup:** Remove dead `INSTALL_PYTHON` build arg. Python 3 + pip + venv have been unconditionally installed in the base layer since mempalace was added; the flag was a no-op. Users should use `uv` (pre-installed) or `uvx` for Python tooling.
+- **Fix:** `mempalace init` in `entrypoint-user.sh` now uses `--yes` for non-interactive operation. Previously the command prompted the user (`Your choice [enter/edit/add]:`) on first container start, which either hung or printed prompts into the user's terminal. The init is still gated by `[ ! -d "$PALACE_DIR/palace" ]` so existing palace data from prior versions is preserved untouched on upgrade.
+- **Feature:** MemPalace is now installed via `uv tool install` into an isolated venv at `/opt/uv-tools/mempalace/`, reached through a new `/usr/local/bin/mempalace-mcp-server` wrapper. Replaces the previous `pip install --break-system-packages` approach — removes the PEP 668 workaround and keeps mempalace deps out of system Python site-packages. The wrapper is what `generate-config.py` now references in the auto-generated `opencode.json`. Users with custom `opencode.json` files should update their mempalace MCP command from `["python3", "-m", "mempalace.mcp_server"]` to `["mempalace-mcp-server"]`.
+- **Feature:** New `INSTALL_MEMPALACE` build arg (default `true`). Rebuild with `--build-arg INSTALL_MEMPALACE=false` to shave ~300 MB off the image when local AI memory isn't needed.
+- **Refactor:** `opencode.json` generation extracted from `entrypoint-user.sh` into a standalone Python script at `/usr/local/lib/opencode-devbox/generate-config.py`. Easier to read, test, and extend with new providers. Default models are declared at the top of the script rather than hard-coded in bash heredocs. Reduces `entrypoint-user.sh` from 176 to 97 lines. Behavior is unchanged — the script preserves the critical guarantee of never overwriting an existing `opencode.json`.
+- **Perf:** Container startup avoids the recursive `chown -R` on named volumes that already have correct ownership. A `.devbox-owner` sentinel file written after a successful chown lets subsequent starts short-circuit via a single `cat`. On volumes with thousands of files (nvim plugins, palace data) this cuts multi-second startup costs to milliseconds. If `USER_UID` changes between runs, the sentinel mismatches and the full chown still runs.
+- **CI:** New `validate` workflow runs on every push to main and PR — single-arch amd64 build, smoke test, and DOCKER_HUB.md sync check. Catches broken Dockerfile changes without waiting for a tag push.
+- **CI:** `docker-publish.yml` now smoke-tests each variant on amd64 before the full multi-arch push. A failing smoke test blocks the release.
+- **CI:** Image size is tracked and fails the build if it exceeds thresholds (base: 2500 MB uncompressed, OMOS: 3000 MB). Makes bloat visible rather than silent.
+- **Docs:** `DOCKER_HUB.md` is now auto-generated from `README.md` via `scripts/generate-dockerhub-md.py`. Editing it directly is a mistake — the `--check` step in CI fails if the committed file is out of sync. Section inclusion is controlled by explicit rules (`SECTION_RULES`, `TRIM_SUBSECTIONS`); adding a new section to README forces an explicit keep/drop/replace decision. Keeps the 25 kB Docker Hub limit in sight and eliminates manual sync burden.
+- **Tests:** New `scripts/smoke-test.sh` asserts: (a) all core binaries are runnable and print a version, (b) opencode starts, (c) entrypoint correctly drops to the developer user, (d) `generate-config.py` produces valid JSON with the expected shape, (e) `generate-config.py` never overwrites an existing config, (f) bun is present only in the OMOS variant, (g) image size is under threshold. The smoke test also logs resolved versions of every component as its first step so CI output always records what got baked in.
+- **Versioning:** All GitHub/Gitea-hosted binaries (gosu, fzf, git-lfs, neovim, bat, eza, zoxide, uv, gitea-mcp) and the go.dev-hosted Go toolchain now default to `latest` at build time. Each `*_VERSION` ARG resolves the newest upstream release by reading the `/releases/latest` Location redirect (or the go.dev JSON feed). Previously these were hand-pinned to a specific version, which meant rebuilds didn't pick up upstream CVE fixes until someone remembered to bump the pin. Pinning is still supported — pass `--build-arg NVIM_VERSION=0.12.1` etc. to lock a specific version. Intentionally still pinned: `OPENCODE_VERSION` (drives the image tag), `NODE_VERSION=22` (major only), `DEBIAN_VERSION=trixie-slim` (OS base).
 
 ## v1.14.28b — 2026-04-27