docs(mentions): specify portable mention rules

[loganj]

🤖
Stacked on #258, which is stacked on #257. Review those first. This PR's diff has only the spec, the fixtures, and the tests that run them.

Summary

• Other Buzz clients need a precise way to give the same mention list and send the same tags as this app. This PR writes the mention rules from Ask before mentioning people outside the channel #257 and Make mention choices consistent and stable #258 as a client-independent spec. It adds shared input and output examples ("fixtures") that any client can use to check its implementation.
• It does not change how the app behaves.

What the spec covers

• Who you can mention: channel members, the agents offered in the channel, and, in channels and forums only, people found in the community directory.
• How @ text is read: when completion opens, and when a query with a space is still part of a name.
• Matching and order: members first, then better matches, then the tie-breaks for agents with the same name.
• Space: when Space completes a mention.
• List stability: rows do not move while the list is open. This is desktop behavior and the fixtures do not test it.
• People outside the channel: the send prompt with Do nothing / Invite, or Send anyway. It follows the block/buzz desktop dialog, and the spec names that source file.
• Tags: a p tag notifies a person (a recipient). A two-field mention tag names a person without notifying them (a reference). This is the same format that other Buzz clients already write. No new event kind or NIP.
• It also says what a mention does not do: choosing a person does not give them access, and it does not promise that an agent will answer.

Details

• The spec is in src/bundled/mentions/README.md, next to the plugin that implements the chooser. This is the same layout as the identity-naming spec (docs(names): specify portable identity disambiguation #227). The README links to it.
• mention-rules.fixtures.json has 66 cases: ranking (23), Space (12), query syntax (12), multi-word queries (6), tag writing (7), and tag reading (6). The expected outputs are written by hand. The code under test does not generate them.
• The app's existing test files run every case through the production code: the ranking and Space functions, the @ query parser, the message send path, and the message reader. No existing test changed.

[loganj]

🤖 Closed for now. This will reopen after #257 and #258 are accepted, in case they change. The branch stays pushed.

[loganj]

🤖
Replaced by #343. GitHub could not reopen this PR after its base branch merged.