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/doctor.go

@@ -108,6 +108,8 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	// Register workspace-level checks first (fundamental)
 	d.RegisterAll(doctor.WorkspaceChecks()...)
 
+	d.Register(doctor.NewGlobalStateCheck())
+
 	// Register built-in checks
 	d.Register(doctor.NewTownGitCheck())
 	d.Register(doctor.NewDaemonCheck())