feat: add task note actions

[smatheusblu]

Summary

• add task-note actions for opening an existing note:<path> and creating/opening a generated Markdown note
• add configurable notes_dir with fallback to $NOTES_DIR and then ~/notes
• launch notes in $VISUAL / $EDITOR, update command palette/help docs, and add tests for note targeting and task mutation

Test plan

• cargo fmt --check
• cargo test --quiet
• cargo clippy --all-targets -- -D warnings

Notes

• o only opens an already-linked, existing note and flashes a hint otherwise.
• O creates a generated note when needed, appends the generated note:<path> token to the task, then opens it.