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/doctor/global_state_check.go

@@ -0,0 +1,118 @@
+// ABOUTME: Doctor check for Gas Town global state configuration.
+// ABOUTME: Validates that state directories and shell integration are properly configured.
+
+package doctor
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/steveyegge/gastown/internal/shell"
+	"github.com/steveyegge/gastown/internal/state"
+)
+
+type GlobalStateCheck struct {
+	BaseCheck
+}
+
+func NewGlobalStateCheck() *GlobalStateCheck {
+	return &GlobalStateCheck{
+		BaseCheck: BaseCheck{
+			CheckName:        "global-state",
+			CheckDescription: "Validates Gas Town global state and shell integration",
+		},
+	}
+}
+
+func (c *GlobalStateCheck) Run(ctx *CheckContext) *CheckResult {
+	result := &CheckResult{
+		Name:   c.Name(),
+		Status: StatusOK,
+	}
+
+	var details []string
+	var warnings []string
+	var errors []string
+
+	s, err := state.Load()
+	if err != nil {
+		if os.IsNotExist(err) {
+			result.Message = "Global state not initialized"
+			result.FixHint = "Run: gt enable"
+			result.Status = StatusWarning
+			return result
+		}
+		result.Message = "Cannot read global state"
+		result.Details = []string{err.Error()}
+		result.Status = StatusError
+		return result
+	}
+
+	if s.Enabled {
+		details = append(details, "Gas Town: enabled")
+	} else {
+		details = append(details, "Gas Town: disabled")
+		warnings = append(warnings, "Gas Town is disabled globally")
+	}
+
+	if s.Version != "" {
+		details = append(details, "Version: "+s.Version)
+	}
+
+	if s.MachineID != "" {
+		details = append(details, "Machine ID: "+s.MachineID)
+	}
+
+	rcPath := shell.RCFilePath(shell.DetectShell())
+	if hasShellIntegration(rcPath) {
+		details = append(details, "Shell integration: installed ("+rcPath+")")
+	} else {
+		warnings = append(warnings, "Shell integration not installed")
+	}
+
+	hookPath := filepath.Join(state.ConfigDir(), "shell-hook.sh")
+	if _, err := os.Stat(hookPath); err == nil {
+		details = append(details, "Hook script: present")
+	} else {
+		if hasShellIntegration(rcPath) {
+			errors = append(errors, "Hook script missing but shell integration installed")
+		}
+	}
+
+	result.Details = details
+
+	if len(errors) > 0 {
+		result.Status = StatusError
+		result.Message = errors[0]
+		result.FixHint = "Run: gt install --shell"
+	} else if len(warnings) > 0 {
+		result.Status = StatusWarning
+		result.Message = warnings[0]
+		if !s.Enabled {
+			result.FixHint = "Run: gt enable"
+		} else {
+			result.FixHint = "Run: gt install --shell"
+		}
+	} else {
+		result.Message = "Global state healthy"
+	}
+
+	return result
+}
+
+func hasShellIntegration(rcPath string) bool {
+	data, err := os.ReadFile(rcPath)
+	if err != nil {
+		return false
+	}
+	return len(data) > 0 && contains(string(data), "Gas Town Integration")
+}
+
+func contains(s, substr string) bool {
+	for i := 0; i <= len(s)-len(substr); i++ {
+		if s[i:i+len(substr)] == substr {
+			return true
+		}
+	}
+	return false
+}