home: add claude-code program with home-manager module

Configures claude-code using the home-manager programs.claude-code module:
- Settings: model, gopls plugin, auto-update disabled, permissions
- Agents: docs-verifier, security-reviewer
- Skills: pr summary generator

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

home/programs/claude-code/skills/pr/SKILL.md

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+---
+name: pr
+description: Generate a PR summary from the current feature branch and copy it to the clipboard.
+argument-hint: [optional: additional context or focus areas]
+user-invocable-only: true
+---
+
+# PR Summary Generator
+
+Generate a concise PR summary comparing the current branch to the main branch (master), and copy it to the clipboard.
+
+## Input
+
+Optional user context: $ARGUMENTS
+
+## Process
+
+1. **Get branch info**: Use git to determine:
+   - Current branch name
+   - Main branch (master)
+   - Verify we're not on master (warn if we are)
+
+2. **Gather commit information**: Use the git-explorer MCP tools:
+   - `commits_between` to get all commits from master to HEAD
+   - `get_commit_info` for each commit to see the full message and changes
+
+3. **Evaluate commit hygiene** (CRITICAL - do this before generating summary):
+
+   Review all commits and check for issues that suggest squashing:
+   - **Fixup commits**: Messages like "fix", "fixup", "oops", "typo", "forgot", "WIP"
+   - **Iterative fixes**: Multiple commits touching the same file for the same logical change
+   - **Broken intermediate states**: Commits that introduce then immediately fix issues
+   - **Noise commits**: Very small changes that don't warrant their own commit
+   - **Related changes split unnecessarily**: Commits that logically belong together
+
+   **If squashing is recommended**: STOP here. Do NOT generate PR summary or copy to clipboard.
+   Instead, respond with a short report:
+   - List which commits should be squashed together
+   - Explain briefly why (e.g., "fixup commit", "same logical change", "WIP")
+   - Suggest the resulting commit message(s)
+
+   **If commits look clean**: Continue to step 4.
+
+4. **Analyze the changes**:
+   - Group commits by topic/area if there are multiple
+   - Note the key files changed
+   - Understand the overall purpose of the branch
+
+5. **Generate the summary**: Create a markdown summary with:
+   - A "Summary" section with 2-4 bullet points describing what changed and why
+   - A "Changes" section listing key files/areas modified
+
+6. **Copy to clipboard**: Pipe the summary to `wl-copy`
+
+## Squash Recommendation Format
+
+When recommending squashing, respond like this:
+
+```
+Before creating a PR, I recommend squashing some commits:
+
+**Squash together:**
+- `abc123` "Add new feature"
+- `def456` "Fix typo in new feature"
+- `ghi789` "Forgot to add import"
+
+→ Suggested message: "Add new feature for X"
+
+**Reason:** These are iterative fixes to the same change.
+
+Run `git rebase -i master` to clean up, then invoke `/pr` again.
+```
+
+## Output Format
+
+```markdown
+## Summary
+
+- First key change or feature
+- Second key change
+- Additional context if needed
+
+## Changes
+
+- `path/to/file.nix` - Brief description of change
+- `path/to/other.nix` - Brief description
+```
+
+## Style Guidelines
+
+- **Concise**: Each bullet point should be one sentence
+- **Focus on "why"**: Explain the purpose, not just what files changed
+- **Group related changes**: Don't list every commit, summarize by area
+- **Technical accuracy**: Use correct terminology for the codebase
+- **No fluff**: Skip obvious statements like "this PR adds..."
+
+## Example
+
+For a branch with commits adding a new monitoring exporter:
+
+```markdown
+## Summary
+
+- Add systemd-exporter to all hosts for tracking service health metrics
+- Configure Prometheus scrape targets for the new exporter
+- Add Grafana dashboard for visualizing systemd unit status
+
+## Changes
+
+- `system/systemd-exporter.nix` - New module enabling systemd-exporter on all hosts
+- `services/monitoring/prometheus.nix` - Add scrape config for systemd-exporter job
+- `services/monitoring/grafana/dashboards/` - New systemd dashboard
+```
+
+## After Generating
+
+1. Confirm the summary was copied to clipboard
+2. Display the summary to the user so they can review it
+3. Mention they can paste it directly into a PR description