chore: set up Entire for AI session checkpointing

[stackptr]

Summary

• Adds pkgs.entire to the devShell and to home.packages for all non-lightweight hosts via a new rc.entire.enable option in modules/home/scm.nix
• Adds .entire/settings.json configuring checkpoints to push to the private stackptr/checkpoints repo instead of this public repo
• Wires Entire's Claude Code hooks into .claude/settings.json (session lifecycle) and .claude/agents/entire-search.md (search subagent)
• Updates the devShell shellHook to run entire enable -y --agent claude-code after pre-commit setup, keeping Entire's git hooks intact if pre-commit ever reinstalls its own hooks
• Updates CLAUDE.md with direnv exec . entire <cmd> pattern for agent sessions in Zed

Testing after merge

1. Switch the system config to get entire on PATH:

   nh darwin switch .#lobtop

2. Verify the install and configuration:

   entire version
   entire status --detailed

3. Start a Claude Code session in this repo, make some changes, and commit. Entire will prompt to link the session to the commit — accept it, then check:

   entire checkpoint list

4. Push and verify the checkpoint branch landed in stackptr/checkpoints (not in this repo) — confirming the checkpoint remote is working.

5. On a second device, pull and run entire checkpoint list to confirm cross-device sync.

Note: There is a known open bug (entireio/cli#800) where checkpoint_remote silently falls back to origin in some cases. Step 4 verifies this is working correctly. The bug appears to affect cross-owner remotes only — since both this repo and the checkpoint repo are under stackptr, it should work.

🤖 Generated with Claude Code

[stackptr]

Note: The devShell includes pkgs.entire unconditionally, so entire enable in the shellHook will actually succeed on lightweight hosts (not just fail silently) — installing git hooks that only function inside the devShell on those hosts. Three options to consider:

1. Leave it as-is — mostly harmless, hooks on lightweight hosts are inert outside the devShell
2. Guard with a settings check — e.g. jq -e '.enabled' .entire/settings.json && entire enable ... to skip on hosts where Entire isn't configured
3. Remove pkgs.entire from the devShell — rely on the system-wide install; lightweight hosts naturally skip entire enable since it won't be on PATH, but direnv exec . entire won't work before the first nh darwin switch