feat(core): tool execute hooks, session instructions, synthetic endpoint

- V2 tool execute hooks: add `ctx.tool.execute.before/after` plugin API,
  core ToolHooks service, and registry wiring so hosted/local tool calls
  run registered before/after hooks; provider-executed calls are excluded.
- Session synthetic endpoint: add `POST /api/session/:sessionID/synthetic`
  with text/description/metadata plus regenerated Promise/Effect/JS client
  surfaces.
- SessionInstructions service: read tool discovers nearby AGENTS.md walking
  up to the Location root (exclusive) and injects them as durable synthetic
  instructions, with lazy history dedup of prior claims.
- to-llm-message: stop forwarding synthetic message metadata to the model so
  bookkeeping annotations stay model-hidden.
- schema changelog entry for synthetic metadata and the metadata leak fix.

packages/server/src/handlers/session.ts

@@ -239,6 +239,27 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
           return HttpApiSchema.NoContent.make()
         }),
       )
+      .handle(
+        "session.synthetic",
+        Effect.fn(function* (ctx) {
+          yield* session.synthetic({
+            sessionID: ctx.params.sessionID,
+            text: ctx.payload.text,
+            description: ctx.payload.description,
+            metadata: ctx.payload.metadata,
+          }).pipe(
+            Effect.catchTag("Session.NotFoundError", (error) =>
+              Effect.fail(
+                new SessionNotFoundError({
+                  sessionID: error.sessionID,
+                  message: `Session not found: ${error.sessionID}`,
+                }),
+              ),
+            ),
+          )
+          return HttpApiSchema.NoContent.make()
+        }),
+      )
       .handle(
         "session.compact",
         Effect.fn(function* (ctx) {