feat: add chat history MCP tools

Adds save/get/list/delete tools for persisting and retrieving agent
chat histories in AMCS.

Changes:
- migrations/018_chat_histories.sql: new chat_histories table with
  indexes on session_id, project_id, channel, agent_id, created_at,
  and FTS over title+summary
- internal/types/extensions.go: ChatMessage and ChatHistory types
- internal/store/chat_histories.go: SaveChatHistory, GetChatHistory,
  GetChatHistoryBySessionID, ListChatHistories, DeleteChatHistory
- internal/tools/chat_history.go: ChatHistoryTool with four handlers
  (save_chat_history, get_chat_history, list_chat_histories,
  delete_chat_history)
- internal/mcpserver/server.go: ChatHistory field in ToolSet,
  registerChatHistoryTools registration function
- internal/app/app.go: wire ChatHistoryTool into ToolSet

migrations/018_chat_histories.sql

@@ -0,0 +1,26 @@
+-- Migration: 018_chat_histories
+-- Adds a dedicated table for saving and retrieving agent chat histories.
+
+CREATE TABLE IF NOT EXISTS chat_histories (
+    id          UUID        PRIMARY KEY DEFAULT gen_random_uuid(),
+    session_id  TEXT        NOT NULL,
+    title       TEXT,
+    channel     TEXT,
+    agent_id    TEXT,
+    project_id  UUID        REFERENCES projects(id) ON DELETE SET NULL,
+    messages    JSONB       NOT NULL DEFAULT '[]',
+    summary     TEXT,
+    metadata    JSONB       NOT NULL DEFAULT '{}',
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_chat_histories_session_id ON chat_histories(session_id);
+CREATE INDEX IF NOT EXISTS idx_chat_histories_project_id ON chat_histories(project_id) WHERE project_id IS NOT NULL;
+CREATE INDEX IF NOT EXISTS idx_chat_histories_channel    ON chat_histories(channel)    WHERE channel IS NOT NULL;
+CREATE INDEX IF NOT EXISTS idx_chat_histories_agent_id   ON chat_histories(agent_id)   WHERE agent_id IS NOT NULL;
+CREATE INDEX IF NOT EXISTS idx_chat_histories_created_at ON chat_histories(created_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_chat_histories_fts
+    ON chat_histories
+    USING GIN (to_tsvector('simple', coalesce(title, '') || ' ' || coalesce(summary, '')));