Refactor save_note to use cache-driven slug collision handling and add tests

[multiplex55]

Motivation

• Remove the pre-save load_notes() call that was used only for slug awareness and rely on the in-memory cache for slug collision context.
• Make slug collision handling deterministic by seeding unique_slug from known cached slugs while excluding the current note’s path/slug.
• Preserve a single authoritative post-write cache refresh via refresh_cache() and add regression tests for common edge cases.

Description

• Replace the pre-save load_notes() with an existing_slugs set derived from CACHE.lock()?.notes that filters out the current note by path/slug.
• Seed deterministic slug generation by calling reset_slug_lookup() and register_slug() for cached slugs and then using unique_slug() when note.slug is empty.
• Add an explicit cached-slug collision guard before writing (if existing_slugs.contains(&slug) && note.path != path && !overwrite { return Ok(false); }) while keeping refresh_cache() as the single refresh after write.
• Add regression tests: save_existing_note_succeeds_without_overwrite, save_as_new_generates_unique_slug_from_cache, and save_note_renames_slug_and_path to cover overwrite, save-as-new slug generation, and rename behavior.

Testing

• Ran cargo fmt --all which succeeded.
• Attempted to run the new tests via cargo test save_note_, but the build aborted due to a missing system dependency required by a transitive crate (alsa/alsa.pc), so the test run could not complete in this environment.
• The added tests are present in src/plugins/note.rs and should pass in an environment with required system libraries installed.

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Codex Task