epic: align skill-router release + self-update with maximal's manifest-based mechanism

[stuffbucket]

Bring the skill-router MCP's self-update UX in line with the manifest-based update mechanism used by stuffbucket/maximal, so users on the primary npm/npx distribution actually learn about and can apply new releases.

Current state (what already works vs. what's broken)

• Release automation — solid, keep as the baseline. Push-to-main auto-patch-bumps and publishes to npm via OIDC (.github/workflows/publish.yml); a manual workflow_dispatch cuts a minor/major bump + GitHub Release with notes (.github/workflows/bump-version.yml); scripts/check-versions.js guards manifest-version drift. No release-automation work is needed for this epic.
• Upgrade detection — exists but broken for the npm channel. The live check uses git ls-remote --tags origin against an install root that, for npx -p @stuffbucket/skills, has no .git — so npx users get no signal. It also compares against git tags, not the npm dist-tag. → fix(mcp): upgrade detection is broken for npm/npx installs — adopt a distribution-correct "latest" source #27
• MCP surface — thin. serverInfo.version is hardcoded "1.0.0"; detection is only a side effect of list_skills; there is no explicit "check for updates" tool. → feat(mcp): expose current version + an explicit update-check tool; fix hardcoded serverInfo.version #28
• In-place upgrade — advertised but unimplemented. update-skills tells users to run ... --update, but no --update flag is handled anywhere; it is a no-op. → feat(mcp): implement the advertised in-place update (--update is a no-op today) #29

The mechanism to mirror (maximal)

maximal's detector (src/lib/update/update-check.ts) is the reference:

• Source of truth is a small JSON fetched over plain HTTP (a CDN-served, channel-keyed manifest), chosen over the GitHub REST API because anonymous REST is 60/h/IP and fails behind shared NAT (docs/decisions/site-runtime-version-manifest.md).
• The check is total / never-throws: offline, non-200, timeout, and malformed all resolve to a coherent status with a diagnostic last_error, preserving last-known.
• In-process TTL cache, fetch timeout, UA header, no auth; hand-rolled semver-precedence compare, zero deps.
• Surfaced notify-only (no auto-install).

For the MCP, the portable adaptation is: read the current version from the package package.json, take "latest" from the npm registry (works for every install type), and keep maximal's cache + never-throws + semver-compare shape.

Work items

• fix(mcp): upgrade detection is broken for npm/npx installs — adopt a distribution-correct "latest" source #27 — Fix detection for npm/npx installs; adopt a distribution-correct "latest" source (npm registry or a static manifest).
• feat(mcp): expose current version + an explicit update-check tool; fix hardcoded serverInfo.version #28 — Expose the real version + an explicit check_updates MCP tool; fix hardcoded serverInfo.version.
• feat(mcp): implement the advertised in-place update (--update is a no-op today) #29 — Implement (or correctly document) the advertised in-place --update upgrade.

Suggested order

#27 (the engine) → #28 (the surface, depends on #27) → #29 (the apply step). #28 and #29 can proceed in parallel once #27 lands.