[tui] GoToMode hook and state machine

[roninjin10]

GoToMode hook and state machine

Implement the go-to mode state machine as a reusable hook.

1. useGoToMode(): Returns { active, activate(), cancel(), destinationHints: GoToHint[], error: string|null }
2. State machine: Inactive → press g → Active (1500ms timer) → second key resolves/cancels → Inactive
3. GoToHint[]: Static array of { key, destination, requiresRepo }
4. Timeout: 1500ms from activation, cleared on any second key or cancel
5. Timer cleanup: useEffect cleanup clears timeout on unmount
6. Error state: 'No repository in context' displayed for 2000ms then auto-clears

Files: apps/tui/src/hooks/useGoToMode.ts

Closes #123

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Auto-generated by smithers workflow
Domain: Codeplane TUI
Ticket: tui-nav-chrome-eng-05