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

internal/types/types.go

@@ -36,8 +36,9 @@ type Issue struct {
 	CreatedAt   time.Time  `json:"created_at"`
 	CreatedBy   string     `json:"created_by,omitempty"` // Who created this issue (GH#748)
 	UpdatedAt   time.Time  `json:"updated_at"`
-	ClosedAt    *time.Time `json:"closed_at,omitempty"`
-	CloseReason string     `json:"close_reason,omitempty"` // Reason provided when closing
+	ClosedAt        *time.Time `json:"closed_at,omitempty"`
+	CloseReason     string     `json:"close_reason,omitempty"`      // Reason provided when closing
+	ClosedBySession string     `json:"closed_by_session,omitempty"` // Claude Code session that closed this issue
 
 	// ===== External Integration =====
 	ExternalRef *string `json:"external_ref,omitempty"` // e.g., "gh-9", "jira-ABC"