Follow-up: move session.rename command registration into use-session-commands

[anduimagui]

Heads up from #15567: we registered session.rename in packages/app/src/pages/session/message-timeline.tsx to keep that PR scoped and unblock desktop menu wiring quickly.

Would be nice to do a small follow-up to move that command registration into packages/app/src/pages/session/use-session-commands.tsx (where the other session command IDs already live), so command discovery/ownership is in one place.

Current state from #15567:

• Desktop menu now triggers session.rename
• App command exists, but it’s registered in the timeline component because that file owns openTitleEditor + title edit state

Follow-up idea:

• Keep rename UI state where it belongs
• Pass a rename action callback (or lift a small bit of state) so use-session-commands.tsx can own the session.rename registration

Not urgent, just cleanup for consistency and maintainability.

Refs:

• PR: feat(desktop): add Rename Session menu shortcut #15567
• Current command registration location: packages/app/src/pages/session/message-timeline.tsx
• Preferred command registration location: packages/app/src/pages/session/use-session-commands.tsx

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