feat(beads): Capture session_id in issue close for CV attribution

Pass CLAUDE_SESSION_ID environment variable to bd close for work attribution
tracking, enabling queries like "what work did this session do?" for entity
CV building (per decision 009-session-events-architecture.md).

Changes:
- beads.Close() and CloseWithReason() now pass --session to bd close
- Updated all direct exec.Command("bd", "close"...) calls:
  - internal/mail/mailbox.go - closeInDir()
  - internal/cmd/swarm.go - swarm land and cancel
  - internal/cmd/hook.go - auto-replace completed beads
  - internal/cmd/synthesis.go - convoy close
  - internal/cmd/crew_lifecycle.go - workspace removal
  - internal/cmd/polecat.go - polecat nuke

The bd CLI already supports --session (or CLAUDE_SESSION_ID env var) so
this change ensures consistent session tracking across all gt close paths.

Fixes: gt-nvz8b

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

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

internal/mail/mailbox.go

@@ -313,7 +313,12 @@ func (m *Mailbox) markReadBeads(id string) error {
 
 // closeInDir closes a message in a specific beads directory.
 func (m *Mailbox) closeInDir(id, beadsDir string) error {
-	cmd := exec.Command("bd", "close", id)
+	args := []string{"close", id}
+	// Pass session ID for work attribution if available
+	if sessionID := os.Getenv("CLAUDE_SESSION_ID"); sessionID != "" {
+		args = append(args, "--session="+sessionID)
+	}
+	cmd := exec.Command("bd", args...)
 	cmd.Dir = m.workDir
 	cmd.Env = append(cmd.Environ(), "BEADS_DIR="+beadsDir)