A test proves watching a milestone: a person in a repository they can read may, an agent and a stranger may not, and no event is appended #7525

Intent: a watch stays the person's own state, as src/actions/watches.ts promises, and this is proven. No test imports the module directly.

Done:

• [ ] A new test/watch-action.test.ts covers watchMilestone. A person who can read the repository starts and stops watching, and isWatching follows. An agent gets forbidden, a guest unauthorized, and someone who cannot read the repository or names an unknown milestone not_found. No event is appended by either a start or a stop, which the test checks against the bus.
• [ ] If any of these fail, fix src/actions/watches.ts, and the pull says what was wrong.

Files: a new test, and src/actions/watches.ts only if the test finds a fault.