feat(claude): add CLAUDE.md, commit command, and update permissions

Add home-manager managed CLAUDE.md with commit behavior instructions,
a /commit slash command for conventional commits workflow, and expand
allowed Bash permissions to include nix commands. Remove inline
customInstructions in favor of CLAUDE.md file.

home.nix

@@ -177,6 +177,46 @@
   programs.neovim-kickstart.enable = true;
 
   # Claude Code configuration
+  home.file.".claude/CLAUDE.md" = {
+    text = ''
+      ## Commit Behavior
+
+      After completing each logical unit of work, use the /commit skill to stage
+      and commit changes before proceeding to the next task. Do not batch unrelated
+      changes into a single commit. Do not wait to be asked.
+    '';
+    force = true;
+  };
+
+  home.file.".claude/commands/commit.md" = {
+    text = ''
+      ---
+      description: Stage and commit changes as a conventional commit with one logical unit per commit
+      ---
+
+      Review the current working tree with `git status` and `git diff --staged` and `git diff`.
+
+      Then do the following:
+
+      1. Identify the smallest logical unit of change present. If multiple unrelated changes exist, only stage and commit one unit at a time — leave the rest unstaged.
+
+      2. Stage only the files (or hunks via `git add -p`) relevant to that logical unit.
+
+      3. Write a commit message following the Conventional Commits format:
+         <type>(<scope>): <description>
+
+         Types: feat, fix, docs, style, refactor, perf, test, chore, ci
+         - scope is optional but use it when the change is clearly scoped to a module, package, or domain
+         - description: lowercase, imperative mood, no trailing period, max ~72 chars
+         - if the change warrants a body, add it after a blank line
+
+      4. Run `git commit` with that message. Never use `--no-verify`.
+
+      5. Report what was committed and what (if anything) remains uncommitted.
+    '';
+    force = true;
+  };
+
   home.file.".claude/settings.json" = {
     text = builtins.toJSON {
       permissions = {
@@ -184,9 +224,11 @@
           "Edit"
           "Write"
           "Bash(git:*)"
+          "Bash(nix-shell:*)"
+          "Bash(nix eval:*)"
+          "Bash(nix flake check:*)"
          ];
       };
-      customInstructions = "When working in a git repository, always use conventional commits format (e.g., feat:, fix:, docs:, style:, refactor:, test:, chore:) for commit messages. Do not include the Claude Code signature or Co-Authored-By line in commit messages.";
       alwaysThinkingEnabled = true;
       includeCoAuthoredBy = false;
     };