v2.1.0: symlink OMOS bundled skills from image, bump opencode 1.17.4->1.17.5

Deploy the five oh-my-opencode-slim bundled skills (clonedeps, codemap,
deepwork, oh-my-opencode-slim, simplify) by symlinking them from the image
path into ~/.agents/skills/ on every container start, instead of the
installer copying them into the persistent config volume on first run only.
Image-sourced links mean 'docker compose pull' + recreate refreshes the
skills with no installer run and no config reset; the old copy-on-first-run
froze them in the volume forever.

- entrypoint-user.sh: new non-fatal OMOS bundled-skills reconcile block
  (runs after skillset deploy so OMOS wins the simplify collision; absolute
  symlinks; gated by OMOS_SKILLS, now independent of ENABLE_OMOS). Both
  installer calls now pass --skills=no. One-time migration backs up (never
  deletes) frozen real copies in ~/.config/opencode/skills/ to .bak.<epoch>.
- scripts/smoke-test.sh: assert the bundled-skills source path on omos.
- Bump OPENCODE_VERSION 1.17.4 -> 1.17.5.
- Versioning: document the move to independent image semver (v2.0.0 was the
  decouple point), mirroring pi-devbox. README/AGENTS/.env.example/CHANGELOG
  updated; new docs/omos-skills.md.

scripts/smoke-test.sh

@@ -180,6 +180,15 @@ if [ "$VARIANT" = "omos" ]; then
                "NPM_CONFIG_PREFIX=/usr npm ls -g --depth=0 2>/dev/null | grep oh-my-opencode-slim" \
                "$EXPECTED_OMOS_VERSION"
   fi
+  # OMOS bundled-skills SOURCE must be present at the fixed image path that
+  # entrypoint-user.sh symlinks into ~/.agents/skills/ on container start. If
+  # upstream restructures the package (moves src/skills), the runtime symlinks
+  # would dangle SILENTLY and the skills would just disappear — assert the
+  # source here so that breakage fails the build loudly instead. We check the
+  # source (not the runtime symlinks) because smoke tests run with
+  # --entrypoint="" and never execute entrypoint-user.sh.
+  run "omos bundled-skills source" \
+      "for n in clonedeps codemap deepwork oh-my-opencode-slim simplify; do test -d /usr/lib/node_modules/oh-my-opencode-slim/src/skills/\$n || exit 1; done && echo ok"
 else
   if docker run --rm --entrypoint="" "$IMAGE" sh -c "command -v bun" >/dev/null 2>&1; then
     fail "bun should NOT be in base image but was found"