phase 1: SQLite session database with FTS5

Port of Hermes's hermes_state.py. Persists session transcripts
in .dirge/sessions/state.db with the same schema: sessions table
+ messages table + messages_fts FTS5 virtual table with content
sync triggers.

Features:
- SessionDb::open() with WAL mode + DELETE fallback for NFS/SMB
- Schema versioning with forward migrations (v1 baseline)
- insert_session / insert_message with message count tracking
- search_messages with FTS5 full-text search + optional role filter
- list_sessions_rich with source exclusion (review-fork, etc.)
- set_parent_session for compression-driven session splitting
- resolve_parent for walking lineage chain to root
- get_anchored_view for ±N message windows around an anchor

9 tests: CRUD, FTS5 search, role filter, source exclusion,
session splitting, parent resolution, anchored message windows.

Added rusqlite with bundled SQLite feature.

Author: Yogthos

Cargo.lock

@@ -121,6 +121,7 @@ html2text = "0.17"
 indexmap = "2"
 lsp-types = { version = "0.97", optional = true }
 tree-sitter-elixir = { version = "0.3.5", optional = true }
+rusqlite = { version = "0.31", features = ["bundled"] }
 
 [dev-dependencies]
 tokio = { version = "1", features = ["test-util"] }

Cargo.toml

@@ -12,3 +12,4 @@ pub mod acp;
 
 pub mod dirge_paths;
 pub mod memory;
+pub mod session_db;