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/rpc/protocol.go

@@ -160,6 +160,7 @@ type UpdateArgs struct {
 type CloseArgs struct {
 	ID          string `json:"id"`
 	Reason      string `json:"reason,omitempty"`
+	Session     string `json:"session,omitempty"`      // Claude Code session ID that closed this issue
 	SuggestNext bool   `json:"suggest_next,omitempty"` // Return newly unblocked issues (GH#679)
 }