MCP `preview` says whether the environment is serving: running, starting, failed with its error, not running or gone #8159

Intent: an agent handed a preview URL learns what a person sees there, in the same words, without fetching the page.

Today preview (src/http/mcp/tools/preview.ts, src/apps/preview_view.ts) answers the app, environment, pull, check and who is working, but not whether anything is serving.

Done looks like:

• [ ] preview adds serving: { state: "running"|"starting"|"failed"|"not_running"|"gone", says, runUrl }. says is the same sentence the proxy's not-serving page shows (one shared words module), and runUrl is set as the page's run link is.
• [ ] The gone/not-running rule is the one from the issue this waits on: an open pull is never gone.
• [ ] docs/MCP.md's preview row names the field. A test per state.

Files: src/apps/preview_view.ts, src/http/mcp/tools/preview.ts, docs/MCP.md, a test. Risk: the error text may carry a path from the build log; pass it through the same redaction the page uses.

Parked by the crew: the check stayed red through 2 automatic tries: a page that answers 200 names its canonical address, equal to og:url; a 404 names none (pull #8224 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.