refactor(beads,mail): split large files into focused modules

Break down monolithic beads.go and mail.go into smaller, single-purpose files:

beads package:
- beads_agent.go: Agent-related bead operations
- beads_delegation.go: Delegation bead handling
- beads_dog.go: Dog pool operations
- beads_merge_slot.go: Merge slot management
- beads_mr.go: Merge request operations
- beads_redirect.go: Redirect bead handling
- beads_rig.go: Rig bead operations
- beads_role.go: Role bead management

cmd package:
- mail_announce.go: Announcement subcommand
- mail_check.go: Mail check subcommand
- mail_identity.go: Identity management
- mail_inbox.go: Inbox operations
- mail_queue.go: Queue subcommand
- mail_search.go: Search functionality
- mail_send.go: Send subcommand
- mail_thread.go: Thread operations

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

internal/beads/beads_rig.go

@@ -0,0 +1,117 @@
+// Package beads provides rig identity bead management.
+package beads
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// RigFields contains the fields specific to rig identity beads.
+type RigFields struct {
+	Repo   string // Git URL for the rig's repository
+	Prefix string // Beads prefix for this rig (e.g., "gt", "bd")
+	State  string // Operational state: active, archived, maintenance
+}
+
+// FormatRigDescription formats the description field for a rig identity bead.
+func FormatRigDescription(name string, fields *RigFields) string {
+	if fields == nil {
+		return ""
+	}
+
+	var lines []string
+	lines = append(lines, fmt.Sprintf("Rig identity bead for %s.", name))
+	lines = append(lines, "")
+
+	if fields.Repo != "" {
+		lines = append(lines, fmt.Sprintf("repo: %s", fields.Repo))
+	}
+	if fields.Prefix != "" {
+		lines = append(lines, fmt.Sprintf("prefix: %s", fields.Prefix))
+	}
+	if fields.State != "" {
+		lines = append(lines, fmt.Sprintf("state: %s", fields.State))
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// ParseRigFields extracts rig fields from an issue's description.
+func ParseRigFields(description string) *RigFields {
+	fields := &RigFields{}
+
+	for _, line := range strings.Split(description, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+
+		colonIdx := strings.Index(line, ":")
+		if colonIdx == -1 {
+			continue
+		}
+
+		key := strings.TrimSpace(line[:colonIdx])
+		value := strings.TrimSpace(line[colonIdx+1:])
+		if value == "null" || value == "" {
+			value = ""
+		}
+
+		switch strings.ToLower(key) {
+		case "repo":
+			fields.Repo = value
+		case "prefix":
+			fields.Prefix = value
+		case "state":
+			fields.State = value
+		}
+	}
+
+	return fields
+}
+
+// CreateRigBead creates a rig identity bead for tracking rig metadata.
+// The ID format is: <prefix>-rig-<name> (e.g., gt-rig-gastown)
+// Use RigBeadID() helper to generate correct IDs.
+// The created_by field is populated from BD_ACTOR env var for provenance tracking.
+func (b *Beads) CreateRigBead(id, title string, fields *RigFields) (*Issue, error) {
+	description := FormatRigDescription(title, fields)
+
+	args := []string{"create", "--json",
+		"--id=" + id,
+		"--title=" + title,
+		"--description=" + description,
+		"--labels=gt:rig",
+	}
+
+	// Default actor from BD_ACTOR env var for provenance tracking
+	if actor := os.Getenv("BD_ACTOR"); actor != "" {
+		args = append(args, "--actor="+actor)
+	}
+
+	out, err := b.run(args...)
+	if err != nil {
+		return nil, err
+	}
+
+	var issue Issue
+	if err := json.Unmarshal(out, &issue); err != nil {
+		return nil, fmt.Errorf("parsing bd create output: %w", err)
+	}
+
+	return &issue, nil
+}
+
+// RigBeadIDWithPrefix generates a rig identity bead ID using the specified prefix.
+// Format: <prefix>-rig-<name> (e.g., gt-rig-gastown)
+func RigBeadIDWithPrefix(prefix, name string) string {
+	return fmt.Sprintf("%s-rig-%s", prefix, name)
+}
+
+// RigBeadID generates a rig identity bead ID using "gt" prefix.
+// For non-gastown rigs, use RigBeadIDWithPrefix with the rig's configured prefix.
+func RigBeadID(name string) string {
+	return RigBeadIDWithPrefix("gt", name)
+}