Add sparse checkout to exclude source repo .claude/ directories

When cloning or creating worktrees from repos that have their own .claude/
directory, those settings would override Gas Town's agent settings. This adds
sparse checkout configuration to automatically exclude .claude/ from all
clones and worktrees.

Changes:
- Add ConfigureSparseCheckout() to git.go, called from all Clone/WorktreeAdd methods
- Add IsSparseCheckoutConfigured() to detect if sparse checkout is properly set up
- Add doctor check to verify sparse checkout config (checks config, not symptoms)
- Doctor --fix will configure sparse checkout for repos missing it

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/doctor/sparse_checkout_check.go

@@ -0,0 +1,119 @@
+package doctor
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/steveyegge/gastown/internal/git"
+)
+
+// SparseCheckoutCheck verifies that git clones/worktrees have sparse checkout configured
+// to exclude .claude/ from source repos. This ensures source repo settings don't override
+// Gas Town agent settings.
+type SparseCheckoutCheck struct {
+	FixableCheck
+	rigPath       string
+	affectedRepos []string // repos missing sparse checkout configuration
+}
+
+// NewSparseCheckoutCheck creates a new sparse checkout check.
+func NewSparseCheckoutCheck() *SparseCheckoutCheck {
+	return &SparseCheckoutCheck{
+		FixableCheck: FixableCheck{
+			BaseCheck: BaseCheck{
+				CheckName:        "sparse-checkout",
+				CheckDescription: "Verify sparse checkout is configured to exclude .claude/",
+			},
+		},
+	}
+}
+
+// Run checks if sparse checkout is configured for all git repos in the rig.
+func (c *SparseCheckoutCheck) Run(ctx *CheckContext) *CheckResult {
+	c.rigPath = ctx.RigPath()
+	if c.rigPath == "" {
+		return &CheckResult{
+			Name:    c.Name(),
+			Status:  StatusError,
+			Message: "No rig specified",
+		}
+	}
+
+	c.affectedRepos = nil
+
+	// Check all git repo locations
+	repoPaths := []string{
+		filepath.Join(c.rigPath, "mayor", "rig"),
+		filepath.Join(c.rigPath, "refinery", "rig"),
+	}
+
+	// Add crew clones
+	crewDir := filepath.Join(c.rigPath, "crew")
+	if entries, err := os.ReadDir(crewDir); err == nil {
+		for _, entry := range entries {
+			if entry.IsDir() && entry.Name() != "README.md" {
+				repoPaths = append(repoPaths, filepath.Join(crewDir, entry.Name()))
+			}
+		}
+	}
+
+	// Add polecat worktrees
+	polecatDir := filepath.Join(c.rigPath, "polecats")
+	if entries, err := os.ReadDir(polecatDir); err == nil {
+		for _, entry := range entries {
+			if entry.IsDir() {
+				repoPaths = append(repoPaths, filepath.Join(polecatDir, entry.Name()))
+			}
+		}
+	}
+
+	for _, repoPath := range repoPaths {
+		// Skip if not a git repo
+		if _, err := os.Stat(filepath.Join(repoPath, ".git")); os.IsNotExist(err) {
+			continue
+		}
+
+		// Check if sparse checkout is configured (not just if .claude/ exists)
+		if !git.IsSparseCheckoutConfigured(repoPath) {
+			c.affectedRepos = append(c.affectedRepos, repoPath)
+		}
+	}
+
+	if len(c.affectedRepos) == 0 {
+		return &CheckResult{
+			Name:    c.Name(),
+			Status:  StatusOK,
+			Message: "All repos have sparse checkout configured to exclude .claude/",
+		}
+	}
+
+	// Build details with relative paths
+	var details []string
+	for _, repoPath := range c.affectedRepos {
+		relPath, _ := filepath.Rel(c.rigPath, repoPath)
+		if relPath == "" {
+			relPath = repoPath
+		}
+		details = append(details, relPath)
+	}
+
+	return &CheckResult{
+		Name:    c.Name(),
+		Status:  StatusError,
+		Message: fmt.Sprintf("%d repo(s) missing sparse checkout configuration", len(c.affectedRepos)),
+		Details: details,
+		FixHint: "Run 'gt doctor --fix' to configure sparse checkout",
+	}
+}
+
+// Fix configures sparse checkout for affected repos to exclude .claude/.
+func (c *SparseCheckoutCheck) Fix(ctx *CheckContext) error {
+	for _, repoPath := range c.affectedRepos {
+		if err := git.ConfigureSparseCheckout(repoPath); err != nil {
+			relPath, _ := filepath.Rel(c.rigPath, repoPath)
+			return fmt.Errorf("failed to configure sparse checkout for %s: %w", relPath, err)
+		}
+	}
+	return nil
+}