Add Nix flake for mcp and mcp-agent

Exposes two packages:
- default (mcp CLI) for operator workstations
- mcp-agent for managed nodes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/sqlite/fcntl.go

@@ -0,0 +1,43 @@
+// Copyright 2024 The Sqlite Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sqlite // import "modernc.org/sqlite"
+
+import (
+	"unsafe"
+
+	"modernc.org/libc"
+	sqlite3 "modernc.org/sqlite/lib"
+)
+
+// Access to sqlite3_file_control
+type FileControl interface {
+	// Set or query SQLITE_FCNTL_PERSIST_WAL, returns set mode or query result
+	FileControlPersistWAL(dbName string, mode int) (int, error)
+}
+
+var _ FileControl = (*conn)(nil)
+
+func (c *conn) FileControlPersistWAL(dbName string, mode int) (int, error) {
+	pi32 := c.tls.Alloc(4)
+	defer c.tls.Free(4)
+
+	*(*int32)(unsafe.Pointer(pi32)) = int32(mode)
+	err := c.fileControl(dbName, sqlite3.SQLITE_FCNTL_PERSIST_WAL, pi32)
+	return int(*(*int32)(unsafe.Pointer(pi32))), err
+}
+
+func (c *conn) fileControl(dbName string, op int, pArg uintptr) error {
+	zDbName, err := libc.CString(dbName)
+	if err != nil {
+		return err
+	}
+	defer c.free(zDbName)
+
+	if rc := sqlite3.Xsqlite3_file_control(c.tls, c.db, zDbName, int32(op), pArg); rc != sqlite3.SQLITE_OK {
+		return c.errstr(rc)
+	}
+
+	return nil
+}