chore: update CLAUDE.md

[stackptr]

Add several rules and patterns to improve consistency and reduce trial-and-error in future sessions.

Changes:

• Branch naming convention with slug length guidance
• PR title format and description expectations
• gt create <name> rule to avoid auto-generated branch names
• gt modify as the correct way to amend branches
• lib.mkForce vs lib.mkDefault explanation
• Darwin eval path alongside NixOS eval path
• agenix workflow: adding, editing, and rekeying secrets
• Environment awareness: check current host before running host-targeting commands
• Learning protocol: proactively ask whether trial-and-error findings should be saved to CLAUDE.md

Test:

• Eval check: nix-flake eval nixosConfigurations.glyph.config.system.build.toplevel.drvPath

[stackptr]

• chore: update CLAUDE.md #438 👈 (View in Graphite)
• main

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