feat(harness): keep input live during a turn; Escape cancels, typing steers

[mike-diff]

Fixes #25 and #26.

Root cause (one bug behind both issues)

During a turn the main goroutine was inside runTurn/drive, so nothing read stdin: the input bar froze until the turn finished. Ctrl+C only ever worked because it rode a separate path (SIGINT), which is exactly why it was flaky and undebuggable.

Fix

A single pump goroutine owns stdin and feeds a select-able channel, so the editor stays live while the turn runs in a worker goroutine. While the agent works you can:

• Type a message + Enter to queue a steer. It's injected as a user turn at the next iteration boundary (the format-safe point, since the provider requires alternating roles), so it steers the next step without corrupting an in-flight reply.
• Press Escape to cancel the turn (resolves [feature] Make Escape the interrupt key #26; reliable now because it's a keystroke, not a signal).

Ctrl+C is reserved for quitting (twice within two seconds), as requested in #26.

How it works

pump() is the only reader of the terminal; every other consumer (readLineMode, readCSI, readPaste, Select, ReadKey, escIsBare) reads decoded runes from the channel. readLineMode gains a turn-attend mode that selects on a keystroke or the turn's done channel, so it stops attending the instant the turn ends without swallowing a keystroke meant for the next prompt. The worker streams the response above the footer while the editor's input row stays live below it; both render under the existing footer mutex.

Scope (called out for review)

• Live input covers the default free-running posture. With -ask, the gate reads approvals mid-turn from the same keyboard, so that posture stays synchronous (Ctrl+C quits). A gate mod (executable) does not read stdin, so it keeps live input.
• Queued steers land at iteration boundaries, not mid-tool-chain (a user message can't be injected between tool rounds without breaking the alternating-role format).
• I could not interactively TTY-test in the dev sandbox (no controlling terminal), so the logic is unit-tested and the terminal plumbing wants a smoke test (type while it works; Enter to steer; Escape to cancel; Ctrl+C twice to quit).

Tests

• TestAttendTurnQueuesAndCancels — drives the live editor: Enter queues, bare Escape cancels.
• TestDriveInjectsQueuedSteer — a queued message is injected as a user turn and acted on in place of the judge.
• TestInterruptsQueue — drain returns and clears (a steer is consumed once).
• Existing e2e drivers keep passing (the pump initializes lazily for test-built consoles).

gofmt/vet clean, full suite + -race on harness/ and agent/ green.