feat: complete bd-kwro messaging & knowledge graph epic

- Add bd cleanup --ephemeral flag for transient message cleanup (bd-kwro.9)
- Add Ephemeral filter to IssueFilter type
- Add ephemeral filtering to SQLite storage queries

Tests (bd-kwro.10):
- Add internal/hooks/hooks_test.go for hook system
- Add cmd/bd/mail_test.go for mail commands
- Add internal/storage/sqlite/graph_links_test.go for graph links

Documentation (bd-kwro.11):
- Add docs/messaging.md for full messaging reference
- Add docs/graph-links.md for graph link types
- Update AGENTS.md with inter-agent messaging section
- Update CHANGELOG.md with all bd-kwro features

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

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

internal/types/types.go

@@ -389,6 +389,9 @@ type IssueFilter struct {
 
 	// Tombstone filtering (bd-1bu)
 	IncludeTombstones bool // If false (default), exclude tombstones from results
+
+	// Ephemeral filtering (bd-kwro.9)
+	Ephemeral *bool // Filter by ephemeral flag (nil = any, true = only ephemeral, false = only non-ephemeral)
 }
 
 // SortPolicy determines how ready work is ordered