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/storage/sqlite/migrations/034_closed_by_session_column.go

@@ -0,0 +1,34 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateClosedBySessionColumn adds the closed_by_session column to the issues table.
+// This tracks which Claude Code session closed the issue, enabling work attribution
+// for entity CV building. See Gas Town decision 009-session-events-architecture.md.
+func MigrateClosedBySessionColumn(db *sql.DB) error {
+	// Check if column already exists
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'closed_by_session'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check closed_by_session column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	// Add the closed_by_session column
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN closed_by_session TEXT DEFAULT ''`)
+	if err != nil {
+		return fmt.Errorf("failed to add closed_by_session column: %w", err)
+	}
+
+	return nil
+}