Add a notification copy style guide

[johnpooch]

What this PR does

The 23 specs in service/notification/registry.py disagree on title, capitalisation, tense, grammatical person and actor naming. There is no written guidance, so each new spec re-decides — and a player in five games receiving "Civil Disorder / France entered civil disorder" cannot tell which game it came from.

Adds .claude/rules/backend/notification-copy.md, scoped by frontmatter to service/notification/registry.py so it loads when a spec is edited, and referenced from .claude/rules/backend/index.md. It states the rules a spec author follows:

• The title is the game name; an event that needs naming goes in the body's first clause.
• Title Case for titles, sentence case for bodies.
• Calls to action are a field (email_link_text), never prose in the body.
• Present perfect for events that just happened.
• Second person only when the event is about the recipient; anonymised twins stay third person.
• Describe what happened, not what the player failed to do.
• One actor-rendering helper with one anonymity check, used everywhere.
• No placeholder strings in user-facing copy — omit the clause instead.
• link = None is a recorded exception with a stated reason, not a per-spec judgement call.

Per the guidance rule in CLAUDE.md, it states rules and points at the registry rather than inventorying the specs. References are by file and symbol rather than file:line, so they do not go stale as the registry moves.

Two facts are asserted from the code rather than assumed. get_email_body() defaults to get_body() and notification_email interpolates that same string into both the hidden preview div and the visible paragraph, which is why the guide requires the body to stand alone. Both tap handlers return early without a link — the notificationclick handler in packages/web/public/firebase-messaging-sw.js and the message listener in packages/web/src/messaging-native.ts — which is why a linkless push is a dead notification.

The guide does not rule on the channel_message truncation, since that interacts with the shared email preview text. It does state the email-subject convention, which all five specs declaring Channel.EMAIL already follow.

Rewriting the existing specs to match is #1283, and the linkless-push fix is #1284; neither is in this PR.

Closes #1282

Checklist

• This PR does one thing — no unrelated fixes, refactors, or drive-by cleanups bundled in
• For PRs of any significant complexity: I ran /review-pr against this PR in Claude Code and addressed (or responded to) its findings
• Tests cover the change
• Screenshots embedded in the PR description for any visual changes (see CLAUDE.md)

Documentation only — two markdown files under .claude/rules/, no code changed, so there is nothing to test and no visual change to screenshot. /review-pr has not been run against this branch.

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