Add AgentIdentity type for session name parsing (gt-ov2uv)

Create AgentIdentity type to parse and construct session names, replacing
duplicated logic in sling.go and handoff.go.

- Add internal/session/identity.go with AgentIdentity type
- ParseSessionName handles: mayor, deacon, witness, refinery, crew, polecat
- SessionName() reconstructs valid tmux session name
- Address() returns mail-style address (e.g., "gastown/crew/max")
- GTRole() returns GT_ROLE env var format
- Update sling.go:sessionToAgentID to use ParseSessionName
- Update handoff.go:sessionToGTRole to use ParseSessionName
- Add comprehensive unit tests with round-trip verification

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

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

internal/session/identity.go

@@ -0,0 +1,144 @@
+// Package session provides polecat session lifecycle management.
+package session
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Role represents the type of Gas Town agent.
+type Role string
+
+const (
+	RoleMayor    Role = "mayor"
+	RoleDeacon   Role = "deacon"
+	RoleWitness  Role = "witness"
+	RoleRefinery Role = "refinery"
+	RoleCrew     Role = "crew"
+	RolePolecat  Role = "polecat"
+)
+
+// AgentIdentity represents a parsed Gas Town agent identity.
+type AgentIdentity struct {
+	Role Role   // mayor, deacon, witness, refinery, crew, polecat
+	Rig  string // empty for mayor/deacon
+	Name string // crew/polecat name (empty for mayor/deacon/witness/refinery)
+}
+
+// ParseSessionName parses a tmux session name into an AgentIdentity.
+//
+// Session name formats:
+//   - gt-mayor → Role: mayor
+//   - gt-deacon → Role: deacon
+//   - gt-<rig>-witness → Role: witness, Rig: <rig>
+//   - gt-<rig>-refinery → Role: refinery, Rig: <rig>
+//   - gt-<rig>-crew-<name> → Role: crew, Rig: <rig>, Name: <name>
+//   - gt-<rig>-<name> → Role: polecat, Rig: <rig>, Name: <name>
+//
+// For polecat sessions without a crew marker, the last segment after the rig
+// is assumed to be the polecat name. This works for simple rig names but may
+// be ambiguous for rig names containing hyphens.
+func ParseSessionName(session string) (*AgentIdentity, error) {
+	if !strings.HasPrefix(session, Prefix) {
+		return nil, fmt.Errorf("invalid session name %q: missing %q prefix", session, Prefix)
+	}
+
+	suffix := strings.TrimPrefix(session, Prefix)
+	if suffix == "" {
+		return nil, fmt.Errorf("invalid session name %q: empty after prefix", session)
+	}
+
+	// Check for global roles first (no rig)
+	switch suffix {
+	case "mayor":
+		return &AgentIdentity{Role: RoleMayor}, nil
+	case "deacon":
+		return &AgentIdentity{Role: RoleDeacon}, nil
+	}
+
+	// Parse rig-based roles
+	parts := strings.Split(suffix, "-")
+	if len(parts) < 2 {
+		return nil, fmt.Errorf("invalid session name %q: expected rig-role format", session)
+	}
+
+	// Check for witness/refinery (suffix markers)
+	if parts[len(parts)-1] == "witness" {
+		rig := strings.Join(parts[:len(parts)-1], "-")
+		return &AgentIdentity{Role: RoleWitness, Rig: rig}, nil
+	}
+	if parts[len(parts)-1] == "refinery" {
+		rig := strings.Join(parts[:len(parts)-1], "-")
+		return &AgentIdentity{Role: RoleRefinery, Rig: rig}, nil
+	}
+
+	// Check for crew (marker in middle)
+	for i, p := range parts {
+		if p == "crew" && i > 0 && i < len(parts)-1 {
+			rig := strings.Join(parts[:i], "-")
+			name := strings.Join(parts[i+1:], "-")
+			return &AgentIdentity{Role: RoleCrew, Rig: rig, Name: name}, nil
+		}
+	}
+
+	// Default to polecat: rig is everything except the last segment
+	if len(parts) < 2 {
+		return nil, fmt.Errorf("invalid session name %q: cannot determine rig/name", session)
+	}
+	rig := strings.Join(parts[:len(parts)-1], "-")
+	name := parts[len(parts)-1]
+	return &AgentIdentity{Role: RolePolecat, Rig: rig, Name: name}, nil
+}
+
+// SessionName returns the tmux session name for this identity.
+func (a *AgentIdentity) SessionName() string {
+	switch a.Role {
+	case RoleMayor:
+		return MayorSessionName()
+	case RoleDeacon:
+		return DeaconSessionName()
+	case RoleWitness:
+		return WitnessSessionName(a.Rig)
+	case RoleRefinery:
+		return RefinerySessionName(a.Rig)
+	case RoleCrew:
+		return CrewSessionName(a.Rig, a.Name)
+	case RolePolecat:
+		return PolecatSessionName(a.Rig, a.Name)
+	default:
+		return ""
+	}
+}
+
+// Address returns the mail-style address for this identity.
+// Examples:
+//   - mayor → "mayor"
+//   - deacon → "deacon"
+//   - witness → "gastown/witness"
+//   - refinery → "gastown/refinery"
+//   - crew → "gastown/crew/max"
+//   - polecat → "gastown/polecats/Toast"
+func (a *AgentIdentity) Address() string {
+	switch a.Role {
+	case RoleMayor:
+		return "mayor"
+	case RoleDeacon:
+		return "deacon"
+	case RoleWitness:
+		return fmt.Sprintf("%s/witness", a.Rig)
+	case RoleRefinery:
+		return fmt.Sprintf("%s/refinery", a.Rig)
+	case RoleCrew:
+		return fmt.Sprintf("%s/crew/%s", a.Rig, a.Name)
+	case RolePolecat:
+		return fmt.Sprintf("%s/polecats/%s", a.Rig, a.Name)
+	default:
+		return ""
+	}
+}
+
+// GTRole returns the GT_ROLE environment variable format.
+// This is the same as Address() for most roles.
+func (a *AgentIdentity) GTRole() string {
+	return a.Address()
+}