feat(tui): clear prompt input on double escape

[azizbecha]

Issue for this PR

Closes # — no existing issue for this one. Related: #14037 asked for Claude Code double-esc parity (restore menu) before being stale-closed; this PR covers a different piece, clearing the input. Happy to open an issue first if that's preferred given the feature guardrail in CONTRIBUTING.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Pressing esc twice quickly (within 500ms) clears the prompt input. Today esc does nothing in the prompt while the session is idle, and ctrl+c is the only way to clear it.

It reuses the two patterns that already exist for this:

• the double-press counter that session.interrupt uses for esc-esc while busy (counter in the prompt store + timeout reset), with a footer hint "esc again to clear" after the first press, styled like the existing "esc again to interrupt" hint
• the existing clearPrompt() path, so the cleared draft is appended to prompt history and recoverable with up-arrow, exactly like ctrl+c

The binding is a new configurable keybind input_clear_double (default escape, set "none" to disable) mapped to a new prompt.clear.double command. Its enabled gate makes it mutually exclusive with every other escape claimant: only fires when the session is idle (esc while busy still interrupts and never touches the text), prompt non-empty, normal mode, no autocomplete popup, no dialog. Escape-cancels-pending-leader-sequence behavior is untouched.

How did you verify your code works?

• New integration test packages/tui/test/cli/tui/prompt-clear.test.tsx mounts the real Prompt component with the full provider stack and drives real key events through the keymap: double-esc clears and the draft is restorable from history via up-arrow, the footer hint appears/disappears, a second esc after 600ms does not clear, input_clear_double: "none" disables the feature, and a busy session (via a session.status event) keeps the interrupt behavior with text intact.
• Keybind plumbing test in test/keymap.test.tsx (compiles to escape, disable works).
• bun test in packages/tui: 196 pass, 0 fail. bun run typecheck clean.
• Tested locally in the running TUI.

Screenshots / recordings

Captured frame from the integration test after the first esc press:

┃  this is a draft message for testing
╹▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
   esc again to clear                agents   commands

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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