[Feature] Clear the prompt with Ctrl+C when idle, and make double-Escape consistently open the tree

[ruttybob]

Area: TUI / Coding agent and CLI

Problem

The clear/interrupt/navigation gestures overlap in a state-dependent way that is easy to trip over:

1. When the editor has content and nothing is running, Ctrl+C does nothing useful — it falls through to handleInterruptKey(), shows an exit hint, and a second press exits. There is no single-key way to discard a long draft.

2. Double-Escape is overloaded with invisible state-dependence (armEscapeRepeat(...) in interactive-mode.ts):

   • editor empty or work running → second Escape opens the message tree (/tree selector)
   • editor has content, idle → second Escape calls clearInputBar() and silently deletes the draft

The destructive branch fires exactly when the user has the most to lose. Concrete case from daily use: I am mid-draft on a prompt, then realize I actually want to send it two messages earlier in the conversation — branch from an earlier point. The natural gesture for that is double-Escape to open the tree and navigate. But because my editor has content, the same gesture wipes the draft instead. The only difference between the two outcomes is invisible editor state, and the hotkey guide ("Interrupt response or clear prompt") covers both without distinguishing them.

To be fair to the counter-arguments raised by @alexanderkjeldaas in #1098: traditional Unix TUIs don't use Ctrl+C for text editing, Ctrl+A Ctrl+K already clears the editor, and in a sluggish TUI a repeated Ctrl+C meant as "clear again" can exit the program and lose state. Both concerns are legitimate — opening this discussion to settle the design rather than assume.

Proposed direction

Give "clear" one unambiguous key and free double-Escape for navigation:

State	Ctrl+C	Double-Escape
Editor has content, idle	Clear prompt	Open tree (consistent with the empty-editor state)
Streaming / bash / retry / compaction	Interrupt operation, preserve draft	Open tree
Editor empty, idle	Exit hint → second press exits	Open tree

• Ctrl+C clears the prompt only when both are true: the editor has content, and no operation is running (!hasInterruptibleWork() — no streaming, bash, compaction, retry). When an operation IS running, Ctrl+C still interrupts and preserves the draft (unlike the earlier fix(coding-agent): clear draft input on Ctrl+C #845 approach, which unconditionally cleared and could lose a draft mid-interrupt).
• Double-Escape stops being state-dependent: it opens the tree selector regardless of editor content, so reaching "send this earlier in the tree" never costs the draft.

Alternatives considered

• Ctrl+A Ctrl+K (existing chord): works today, but is a two-chord sequence and undiscoverable
• Keep double-Escape → clear and bind the tree to a dedicated key: preserves current muscle memory, but leaves the destructive state-dependent gesture in place

Additional context

This division comes from pi (earendil-works/pi), the upstream project Prime Agent's TUI is built on: there ctrl+c is bound to app.clear ("Clear editor") and escape to app.interrupt ("Cancel or abort"), so clearing and interrupting are never the same gesture. As a daily pi user, I find this split works well in practice and consider it a good reference point for Prime Agent.

I prototyped the Ctrl+C part in #1098 (closing it now per the new contribution process). Implementation is small: in handleCtrlC(), check editor.getText().length > 0 && !hasInterruptibleWork() before falling through to the exit-hint path, plus hotkey-guide and test updates. The double-Escape change is a tweak of the armEscapeRepeat(...) predicate that currently selects "tree" : "clear". Can rebase onto current main if invited.