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/core/src/plugin.ts

@@ -18,6 +18,7 @@ import { Reference } from "./reference"
 import { SkillV2 } from "./skill"
 import { State } from "./state"
 import { ToolRegistry } from "./tool/registry"
+import { ToolHooks } from "./tool/hooks"
 
 export const ID = Plugin.ID
 export type ID = typeof ID.Type
@@ -165,6 +166,7 @@ export const node = makeLocationNode({
     Reference.node,
     SkillV2.node,
     ToolRegistry.toolsNode,
+    ToolHooks.node,
     PluginRuntime.node,
   ],
 })