feat: sqlite persistence with goose migrations, flashcard schema, zap logger

- Add SQLite as authoritative store alongside Qdrant (best-effort indexing)
- Use goose for embedded SQL migrations
- Schema matches flashcard format: id(INTEGER PK), note, repos, host, tags, user, context, created
- Replace std log with zap structured logger
- Split registerTools into registerStoreKnowledge + registerSearchKnowledge
- Qdrant index uses SQLite rowid directly as point ID (no SHA256 hash)
- Search enriches Qdrant results from SQLite by ID
- Lowercase auth header before prefix check
- Always handle json.MarshalIndent errors
- NixOS: StateDirectory, sqlitePath to /var/lib/st-pierre/st-pierre.sqlite

migrations/000001_create_knowledge.sql

@@ -0,0 +1,14 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS knowledge (
+    id INTEGER PRIMARY KEY,
+    note TEXT NOT NULL,
+    repos TEXT,
+    host TEXT,
+    tags TEXT,
+    "user" TEXT,
+    context TEXT,
+    created TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS knowledge;