feat(core): reload agents from config change feed - #39167

What

Agent source invalidation via config.changes: editing, creating, renaming, or deleting Markdown files under a config root's {agent,agents} or legacy {mode,modes} directories now rebuilds agent state directly, without a config.updated event that descendant edits never trigger. Fifth slice of the config source reload series (#38983, #39131, #39157, #39160).

How

Same seam as the command plugin in #39160, with two refinements from review:

• One merged trigger stream. Stream.merge(sourceChanges, configUpdates) feeds a single debounce and a single runForEach, so filesystem invalidations and structured config.updated reloads are serialized on one fiber — no duplicate load() scans on correlated bursts and no stale-scan overwrite of loaded.documents.
• FSUtil.contains for path containment (Windows-robust, same helper Config's own reconcile uses), driven by one sourceDirectories constant colocated with legacySources instead of a second literal name list.

The predicate matches anything at or under <root>/{agent,agents,mode,modes} with no file-suffix check, so directory-level renames that carry no per-file paths still invalidate. The subscription forks before the initial scan.

New shared test helpers land in test/lib/clock.ts (advance, drain): both Stream.debounce (100ms) and State.reload (500ms batch) register TestClock sleeps from fiber hops a single TestClock.adjust flush can miss, so the helpers alternate real-macrotask settles with adjusts until the asserted condition holds.

Testing

Source-lifecycle matrix using the #39157 fixtures (Config.testLayer + Config.Test.emitChange):

• created / updated / renamed / deleted, plus created in a legacy modes directory (asserts mode: "primary"), each verifying inside the agent.updated subscription so the event is asserted as a read barrier.
• Debounce coalescing: three updates in one window produce exactly one rebuild; a later update produces a second.
• Isolation: updates outside agent source directories (including a commands/ path) produce zero rebuilds, and the feed stays live afterwards.

Red first (all 7 fail without the fix), green with it; stable across 10 consecutive runs. bun test test/config/ passes except the pre-existing PluginSupervisor failure present on clean base; bun typecheck clean.

Follow-ups

• Align the command plugin with the merged-stream + FSUtil.contains shape and migrate command.test.ts's inline clock helpers to test/lib/clock.ts (deferred from feat(core): reload commands from config change feed #39160 to avoid review churn).
• Optional no-op rebuild suppression (deep-equality guard before reload, precedented by Config.reload) — needs its own red test, applies to both plugins.