feat: Set and Forget - Seamless Gas Town Integration (#255)

Adds shell integration for automatic Gas Town context detection.

Features:
- `gt enable` / `gt disable` - Global on/off switch
- `gt shell install|remove|status` - Shell integration management
- `gt rig quick-add [path]` - One-command project setup
- `gt uninstall` - Clean removal with options
- Shell hook auto-sets GT_TOWN_ROOT/GT_RIG on cd

Implementation:
- XDG-compliant state storage (~/.local/state/gastown/)
- Safe RC file manipulation with block markers
- Environment overrides (GASTOWN_DISABLED/ENABLED)
- Doctor check for global state validation

Co-authored-by: Sohail Mohammad <sohailm25@gmail.com>

internal/cmd/rig_detect.go

@@ -0,0 +1,147 @@
+// ABOUTME: Hidden command for shell hook to detect rigs and update cache.
+// ABOUTME: Called by shell integration to set GT_TOWN_ROOT and GT_RIG env vars.
+
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/state"
+	"github.com/steveyegge/gastown/internal/workspace"
+)
+
+var rigDetectCache string
+
+var rigDetectCmd = &cobra.Command{
+	Use:    "detect [path]",
+	Short:  "Detect rig from repository path (internal use)",
+	Hidden: true,
+	Long: `Detect rig from a repository path and optionally cache the result.
+
+This is an internal command used by shell integration. It checks if the given
+path is inside a Gas Town rig and outputs shell variable assignments.
+
+When --cache is specified, the result is written to ~/.cache/gastown/rigs.cache
+for fast lookups by the shell hook.
+
+Output format (to stdout):
+  export GT_TOWN_ROOT=/path/to/town
+  export GT_RIG=rigname
+
+Or if not in a rig:
+  unset GT_TOWN_ROOT GT_RIG`,
+	Args: cobra.MaximumNArgs(1),
+	RunE: runRigDetect,
+}
+
+func init() {
+	rigCmd.AddCommand(rigDetectCmd)
+	rigDetectCmd.Flags().StringVar(&rigDetectCache, "cache", "", "Repository path to cache detection result for")
+}
+
+func runRigDetect(cmd *cobra.Command, args []string) error {
+	checkPath := "."
+	if len(args) > 0 {
+		checkPath = args[0]
+	}
+
+	absPath, err := filepath.Abs(checkPath)
+	if err != nil {
+		return outputNotInRig()
+	}
+
+	townRoot, err := workspace.Find(absPath)
+	if err != nil || townRoot == "" {
+		return outputNotInRig()
+	}
+
+	rigName := detectRigFromPath(townRoot, absPath)
+
+	if rigName != "" {
+		fmt.Printf("export GT_TOWN_ROOT=%q\n", townRoot)
+		fmt.Printf("export GT_RIG=%q\n", rigName)
+	} else {
+		fmt.Printf("export GT_TOWN_ROOT=%q\n", townRoot)
+		fmt.Println("unset GT_RIG")
+	}
+
+	if rigDetectCache != "" {
+		if err := updateRigCache(rigDetectCache, townRoot, rigName); err != nil {
+			fmt.Fprintf(os.Stderr, "warning: could not update cache: %v\n", err)
+		}
+	}
+
+	return nil
+}
+
+func detectRigFromPath(townRoot, absPath string) string {
+	rel, err := filepath.Rel(townRoot, absPath)
+	if err != nil || strings.HasPrefix(rel, "..") {
+		return ""
+	}
+
+	parts := strings.Split(rel, string(filepath.Separator))
+	if len(parts) == 0 || parts[0] == "." {
+		return ""
+	}
+
+	candidateRig := parts[0]
+
+	switch candidateRig {
+	case "mayor", "deacon", ".beads", ".claude", ".git", "plugins":
+		return ""
+	}
+
+	rigConfigPath := filepath.Join(townRoot, candidateRig, "config.json")
+	if _, err := os.Stat(rigConfigPath); err == nil {
+		return candidateRig
+	}
+
+	return ""
+}
+
+func outputNotInRig() error {
+	fmt.Println("unset GT_TOWN_ROOT GT_RIG")
+	return nil
+}
+
+func updateRigCache(repoRoot, townRoot, rigName string) error {
+	cacheDir := state.CacheDir()
+	if err := os.MkdirAll(cacheDir, 0755); err != nil {
+		return err
+	}
+
+	cachePath := filepath.Join(cacheDir, "rigs.cache")
+
+	existing := make(map[string]string)
+	if data, err := os.ReadFile(cachePath); err == nil {
+		lines := strings.Split(string(data), "\n")
+		for _, line := range lines {
+			if idx := strings.Index(line, ":"); idx > 0 {
+				existing[line[:idx]] = line[idx+1:]
+			}
+		}
+	}
+
+	var value string
+	if rigName != "" {
+		value = fmt.Sprintf("export GT_TOWN_ROOT=%q; export GT_RIG=%q", townRoot, rigName)
+	} else if townRoot != "" {
+		value = fmt.Sprintf("export GT_TOWN_ROOT=%q; unset GT_RIG", townRoot)
+	} else {
+		value = "unset GT_TOWN_ROOT GT_RIG"
+	}
+
+	existing[repoRoot] = value
+
+	var lines []string
+	for k, v := range existing {
+		lines = append(lines, k+":"+v)
+	}
+
+	return os.WriteFile(cachePath, []byte(strings.Join(lines, "\n")+"\n"), 0644)
+}