feat(schema): add messaging fields for bd-kwro epic

- Add TypeMessage issue type for inter-agent communication
- Add 6 new Issue fields: Sender, Ephemeral, RepliesTo, RelatesTo,
  DuplicateOf, SupersededBy
- Add 4 new dependency types: replies-to, relates-to, duplicates, supersedes
- Create migration 019_messaging_fields with indexes
- Update all CRUD operations across storage layer
- Fix reset_test.go to use correct function names
- Fix redundant newline lint error in sync.go

Closes: bd-kwro.1

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

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

internal/storage/sqlite/schema.go

@@ -27,6 +27,13 @@ CREATE TABLE IF NOT EXISTS issues (
     deleted_by TEXT DEFAULT '',
     delete_reason TEXT DEFAULT '',
     original_type TEXT DEFAULT '',
+    -- Messaging fields (bd-kwro)
+    sender TEXT DEFAULT '',
+    ephemeral INTEGER DEFAULT 0,
+    replies_to TEXT DEFAULT '',
+    relates_to TEXT DEFAULT '',
+    duplicate_of TEXT DEFAULT '',
+    superseded_by TEXT DEFAULT '',
     CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 );