feat(notifications): customizable comment-notification recipients

[yumike]

What

Adds a code extension seam so a company can replace who is notified about doc comments (e.g. notify a maintainers user-list instead of all members of the owning group), with today's behavior preserved as the default.

How

• CommentRecipientResolver — resolveRecipients(activity) => Promise<string[]> ([] = notify nobody; no null sentinel).
• rwCommentRecipientExtensionPoint — a module-owned extension point with a single setRecipientResolver (throws on double-registration), mirroring Backstage's own setNotificationRecipientResolver. A sibling pluginId: "rw" module registers a resolver; the notifications module reads it in its own init (the framework orders the sibling first).
• DefaultCommentRecipientResolver (exported) — the prior policy (new thread → section owner; reply/resolve → participants). Custom resolvers delegate to it for audiences they don't override.
• CommentNotifier delegates recipient selection to the resolver and is fail-closed: if a resolver throws, the notification is discarded and logged rather than falling back to the broader default audience. Coalescing scope and topic stay tied to the activity kind, not the recipients.
• Re-documents CommentActivity.sectionOwnerRef as a neutral fact (rw-node, doc-only).
• README: a "Customizing who gets notified" section with a worked maintainers example + install-order precondition.

Notes

• Behavior is unchanged for existing installs (no resolver registered → built-in default).
• Recipients-only seam: scope/topic/formatting/actor-exclusion are untouched.
• 27 tests (incl. startTestBackend integration tests proving the override and double-register-throws); typecheck/lint/format clean across all workspaces.

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