refactor(llm): remove GBNF grammar, switch to JSON-envelope extractor

extract_content_tags now generates with grammar=None and parses the
response via a manual brace-counting JSON envelope extractor that
handles Qwen <think>...</think> prefixes and trailing stop tokens.
Five new unit tests. Bumps llama-cpp-2 to 0.1.146. Explicit
features=[] on tauri dependency (no-op).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -41,7 +41,7 @@ magnotia-llm = { path = "../crates/llm" }
 # autostart plugins are desktop-only — gated below under
 # cfg(not(target_os = "android")). The dialog, opener, and notification
 # plugins all support Android natively so live in the unconditional list.
-tauri = { version = "2" }
+tauri = { version = "2", features = [] }
 tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
 # Phase 6 nudges: OS-native notifications via the frontend-owned nudge