refactor(core): use Latch for plugin supervisor ready gate

[kitlangton]

What

The plugin supervisor's flush readiness signal is a hand-rolled reusable gate: a Deferred<void> in a mutable ready.current box, recreated via an isDone check on every update and re-awaited through Effect.suspend. Effect's Latch is exactly this primitive — a valueless, never-failing gate that can close and reopen — so this replaces the machinery with one Latch.

How

packages/core/src/plugin/supervisor.ts:

• ready is now yield* Latch.make() (starts closed, like the fresh deferred).
• Each observed update runs ready.close (idempotent) instead of isDone + recreate; the mutable box disappears.
• Activation catching up to the latest generation runs ready.open instead of Deferred.succeed.
• flush is ready.await directly — no Effect.suspend needed since there is no longer a swapped reference to re-read.

Behavior is unchanged: updates close the gate before the burst coalesces (same "make accepted work visible to flush" point in the stream), and only an activation that has caught up to the last observed generation opens it.

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/plugin.test.ts test/plugin-hooks.test.ts test/session-runner.test.ts (172 pass), and the full bun run test suite (1892 pass / 0 fail).