Support lossless extension shortcut sequences

## Motivation

Extensions can currently register single-key shortcuts, but they cannot safely own a multi-key sequence such as `Ctrl+X`, then `Down`.

This matters for subagent tooling: users can see that child agents are running, but an extension needs a conflict-safe way to open an inspection surface without replacing Senpi's existing `Ctrl+X` copy behavior.

## Requested behavior

- Add a public extension API for ordered shortcut sequences.
- Use a 2,000 ms leader window by default.
- Let extensions provide an optional availability predicate.
- Replay buffered input exactly once on timeout or mismatch.
- Preserve existing single-key shortcuts and immediate `Ctrl+X` copy when no sequence is available.
- Dispose pending timers and registrations during reload or shutdown.

## Why this belongs in Senpi

Senpi owns terminal input dispatch and existing keybindings. Implementing sequence matching in a consumer extension would either swallow input or duplicate host behavior. A host-level API keeps replay, conflict handling, diagnostics, and lifecycle ownership in one place.

Start by tracing Senpi's terminal input dispatch and existing extension shortcut registration, then inspect the current Ctrl+X copy path. The work is done when extensions can register ordered sequences with availability checks, timeout or mismatch replay, preserved single-key behavior, and cleanup during reload or shutdown.

Written by the indexing model from the issue text.