Label and plan refusals on a thread live in one module, and the page and MCP read them from there #5564

For: Changing labels or planning on an issue reads the same from every door, because every door reads it from one place. Today each sentence below is typed out separately in src/web/pages/thread.ts and src/http/mcp.ts (labels, plan), so the next edit to one door makes them drift. This is the fix pull #5504's reviewer asked for, applied to these words.

Done:

• [ ] A new src/actions/thread_words.ts exports these sentences (a const object or small functions, plain data):

- "You need write access to change labels." - "One of those labels no longer exists." - "You need write access, or a place on the crew, to plan here."

• [ ] Each door (src/web/pages/thread.ts and src/http/mcp.ts (labels, plan)) imports them, and none of these sentences is typed out anywhere else under src/ (grep -rn for each finds only the module).
• [ ] Answers are byte-for-byte unchanged: every existing test passes as it is.
• [ ] A new test/thread-words.test.ts triggers one of the refusals through each door and asserts each answers the module's sentence. It fails if a door retypes the words.
• [ ] Before pushing, run test/thread-words.test.ts, the tests of each door you touched and bun x tsc --noEmit. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Files: src/actions/thread_words.ts (new), the doors named, test/thread-words.test.ts. Risk: a pure move; change no wording. Other MCP work edits src/http/mcp.ts in parallel, so keep each edit to the literal's own line.