refactor(core): use Latch for shell output gate

[kitlangton]

What

The shell session's outputDone signal is a pure "output pump drained" gate — a Deferred<void> succeeded on both the success and catch paths of the pump fiber and awaited once in finish. Latch expresses that open-once, never-failing intent directly.

How

packages/core/src/shell.ts:

• outputDone is now Latch.makeUnsafe() (starts closed).
• Both completion sites (normal drain and the catch fallback) use outputDone.open.
• finish waits with outputDone.await.

The value/error-carrying deferreds in the same file (ready, waiter Info resolution) are correct as Deferred and unchanged. No behavior change. Found by a repo-wide audit of Deferred<void>-as-gate patterns; sibling PRs convert the other pure-gate sites.

Testing

• packages/core: bun typecheck; bun test test/shell.test.ts test/shell-cleanup.test.ts test/tool-shell.test.ts (29 pass); full bun run test (1908 tests, 0 fail).