feat(core): make path-local instruction discovery durable

Replace synthetic-message instruction injection with a durable
discovery projection so discovered AGENTS.md files survive compaction,
forks, reverts, and restarts.

- Add the instruction_file table and the durable
  session.instructions.discovered event; projection stores each
  discovered path and content with its owning assistant-message
  boundary and durable discovery order.
- Fold discovered files into the core/instructions source through
  InstructionDiscovery, absorbing SessionInstructions. Completed
  compaction rebaselines restate discovered instructions instead of
  summarizing them away.
- Re-read discovered files live at each observation so mid-session
  edits reach the model; the frozen discovery content stands in only
  when a file becomes unreadable.
- Narrate instruction file changes as per-file deltas via diffByKey,
  restating the full set only for pure reorderings.
- Fork copies discoveries within the inherited transcript, committed
  revert removes discoveries past the revert boundary, and Session
  movement clears them so the destination initializes a complete
  baseline.

packages/schema/test/event-manifest.test.ts

@@ -104,7 +104,7 @@ describe("public event manifest", () => {
         "session.forked.1",
         "session.prompt.promoted.1",
         "session.prompt.admitted.1",
-
+        "session.instructions.discovered.1",
         "session.instructions.updated.1",
         "session.synthetic.1",
         "session.skill.activated.1",