fix(tui): make steer, escape, enter, and double ctrl-c work under extended-keys

[mike-diff]

Closes #30.

The interactive control plane did not reliably drive a running turn: a typed steer only took effect at the next iteration boundary, and Escape, Enter, and Ctrl+C did nothing under terminals using the extended-keys (CSI-u) keyboard protocol (e.g. tmux with extended-keys on). In that mode those keys arrive as key sequences through the input stream rather than a bare byte or an OS signal, so the existing handlers never saw them:

• Escape -> ESC[27u
• Enter -> ESC[57414u (kitty functional) or ESC[13u
• Ctrl+C -> ESC[99;5u (never delivered as SIGINT)

What changed

• steer: typing during a turn now cancels the in-flight turn at once and runs the queued text as the next turn; the queued note is highlighted in cyan. A sticky abort flag closes the between-iteration window where a cancel was previously dropped, so Escape also aborts the judge and worker phases.
• escape: bare-Escape detection keys off the sequence introducer ([/O) rather than a fragile timer, and the CSI-u Escape (ESC[27u) is decoded and routed to cancel.
• enter: the CSI-u Enter (codepoint 13 or kitty functional 57414) is normalized back to a plain Return so the single submit/steer path handles it; a modified Enter (Shift/Alt) still inserts a newline.
• ctrl+c: force-exit no longer blocks on the kill grace period; the CSI-u Ctrl+C (ESC[99;5u) and a raw 0x03 are routed to the same quit path as the OS signal, so a double press exits even under extended-keys.
• /stop: a new command, the typed twin of Escape (aborts the turn, queues nothing).
• The proc tool now threads the turn context so it shares bash's cancellable contract.

Verification

• gofmt, go vet, go build ./..., go test ./... all clean; race detector clean on the touched packages.
• New behavior tests for each path (steer-cancel, sticky-abort gap, escIsBare introducer detection, CSI-u Escape/Enter/Ctrl+C decode, /stop, no-block force-exit), each with a nameable one-line breaker.
• Verified live in tmux (extended-keys on): steer, Escape, Enter, /stop, and double Ctrl+C all confirmed working; the exact key sequences were captured during debugging and pinned by the decode helpers.

agent/ is untouched; all changes live in harness/.