feat: add session_id field to issue close/update mutations (bd-tksk)

Adds closed_by_session tracking for entity CV building per Gas Town
decision 009-session-events-architecture.md.

Changes:
- Add ClosedBySession field to Issue struct
- Add closed_by_session column to issues table (migration 034)
- Add --session flag to bd close command
- Support CLAUDE_SESSION_ID env var as fallback
- Add --session flag to bd update for status=closed
- Display closed_by_session in bd show output
- Update Storage interface to include session parameter in CloseIssue

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/show.go

@@ -209,6 +209,9 @@ var showCmd = &cobra.Command{
 					if issue.CloseReason != "" {
 						fmt.Printf("Close reason: %s\n", issue.CloseReason)
 					}
+					if issue.ClosedBySession != "" {
+						fmt.Printf("Closed by session: %s\n", issue.ClosedBySession)
+					}
 					fmt.Printf("Priority: P%d\n", issue.Priority)
 					fmt.Printf("Type: %s\n", issue.IssueType)
 					if issue.Assignee != "" {
@@ -426,6 +429,9 @@ var showCmd = &cobra.Command{
 			if issue.CloseReason != "" {
 				fmt.Printf("Close reason: %s\n", issue.CloseReason)
 			}
+			if issue.ClosedBySession != "" {
+				fmt.Printf("Closed by session: %s\n", issue.ClosedBySession)
+			}
 			fmt.Printf("Priority: P%d\n", issue.Priority)
 			fmt.Printf("Type: %s\n", issue.IssueType)
 			if issue.Assignee != "" {