Support readable path filters for notes diff

[brownie-ricon]

Problem

While editing a single readable note in a notes-managed repo, the obvious command failed:

notes diff notes/scratch-mise-workbench.md

Observed output:

Error: not a tree-ish ref: notes/scratch-mise-workbench.md
[diff] ERROR task failed

notes diff currently treats positional arguments as refs/ranges. That makes sense for BASE, BASE..HEAD, and BASE...HEAD, but a readable note path is also a natural thing to pass when reviewing one modified note.

Why this would help

The working-tree diff without arguments works, but it shows all readable note changes. During note-only edits, agents often want to inspect exactly one note before notes commit / notes stage.

Desired shape could be either:

notes diff notes/scratch-mise-workbench.md

or, if avoiding ambiguity with refs is preferable:

notes diff -- notes/scratch-mise-workbench.md
notes diff --path notes/scratch-mise-workbench.md

Acceptance sketch

• A readable note path can be used to filter the working-tree readable diff.
• If a positional argument is neither a valid ref/range nor an existing readable note path, the error explains both accepted forms.
• Existing ref/range and --pr behavior remains unchanged.

Provenance

Hit by Brownie while adding a fold scratch-mise-workbench dogfood note on 2026-06-20. Or immediately said this would be nice to support.

[junior-ricon]

Reproduced on 2026-07-14 with released notes v0.8.19 (b27fa7d) while editing one fold note:

$ notes diff independent-then-context-review.md
Error: not a tree-ish ref: independent-then-context-review.md
[diff] ERROR task failed

Running notes diff without arguments correctly rendered the working-tree readable diff.

This also confirms that users naturally try the basename relative to the configured/default notes directory, not only notes/<name>.md. I suggest covering both forms in the eventual path-filter contract, while retaining -- or an explicit path flag as the unambiguous form when a path could collide with a ref.