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/HACKING.md

@@ -0,0 +1,6 @@
+Please do not tag commits unless all builders at
+https://modern-c.appspot.com/-/builder/?importpath=modernc.org%2fsqlite are
+happy.
+
+Since 2024-03-12 it should be no more necessary to manually tag releases at
+all as the repo is now set to be tagged automatically.