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/cmd/synthesis.go

@@ -321,7 +321,11 @@ func runSynthesisClose(cmd *cobra.Command, args []string) error {
 	}
 
 	// Close the convoy
-	closeCmd := exec.Command("bd", "close", convoyID, "--reason=synthesis complete")
+	closeArgs := []string{"close", convoyID, "--reason=synthesis complete"}
+	if sessionID := os.Getenv("CLAUDE_SESSION_ID"); sessionID != "" {
+		closeArgs = append(closeArgs, "--session="+sessionID)
+	}
+	closeCmd := exec.Command("bd", closeArgs...)
 	closeCmd.Dir = townBeads
 	closeCmd.Stderr = os.Stderr