feat: add persistent CPU search engine and CLI with lifecycle tests

crates/searchi-cli/README.md

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+# Searchi CLI
+
+The CLI is a thin adapter over `searchi-core`; it performs no network requests or
+GPU setup. Index only folders you explicitly choose.
+
+```powershell
+cargo run -p searchi-cli -- index --db ./searchi.db --root ./documents
+cargo run -p searchi-cli -- search --db ./searchi.db "termination clause" --limit 10
+cargo run -p searchi-cli -- search --db ./searchi.db "invoice" --json
+cargo run -p searchi-cli -- stats --db ./searchi.db --json
+```
+
+The same arguments work on Linux and macOS. After building, invoke the `searchi`
+binary directly instead of `cargo run -p searchi-cli --`.
+
+`index` creates the database if needed; its parent directory must already exist.
+`search` and `stats` require an existing database so a mistyped path does not
+silently create an empty index. Failures exit with a nonzero status and include
+the affected folder or database path. Result limits must be between 1 and 1000.
+
+`search --json` emits an array of engine results, including the path, filename,
+snippet, ranking score, and match kind. An empty search emits `[]`. `stats --json`
+emits an object with `files` and `content_files`. JSON is the scripting interface;
+the human-readable output may change as preview support develops.
+
+The foundation CLI searches filenames and extracted text. Semantic inference,
+document formats beyond the core extractor, and desktop integration remain
+tracked in the project roadmap; this interface does not fabricate embeddings.