feat(mcp): add describe_tools and annotate_tool functionality

* Implement DescribeTool for listing available MCP tools with annotations.
* Add UpsertToolAnnotation and GetToolAnnotations methods for managing tool notes.
* Create tool_annotations table for storing tool usage notes.

migrations/019_tool_annotations.sql

@@ -0,0 +1,14 @@
+-- Migration: 019_tool_annotations
+-- Adds a table for model-authored usage notes per tool.
+
+create table if not exists tool_annotations (
+  id         bigserial   primary key,
+  tool_name  text        not null,
+  notes      text        not null default '',
+  created_at timestamptz not null default now(),
+  updated_at timestamptz not null default now(),
+  constraint tool_annotations_tool_name_unique unique (tool_name)
+);
+
+grant all on table public.tool_annotations to amcs;
+grant usage, select on sequence tool_annotations_id_seq to amcs;