Changelog entry conventions

Changelog entries follow a consistent, repo-wide convention, since many PRs were flagged for violating it:

1. Placement: New entries always go under ## [Unreleased], under the correct ### Added/### Changed/### Fixed/### Removed subsection matching the nature of the change. Never add or edit content under an already-released version heading — released sections are immutable. If a change has no user-visible effect (internal cleanup, test-tree changes, unused exports with no consumers), it needs no changelog entry at all.

2. Correct package/file: The entry belongs in the CHANGELOG.md of the package whose code actually changed. Don’t bundle entries for unrelated commits/issues together, and don’t let an entry drift into the wrong package file.

3. Attribution for external contributions: Every externally contributed, user-facing fix/feature must credit the PR and author using the exact convention: ([#NNNNN](https://github.com/<org>/<repo>/pull/NNNNN) by [@username](https://github.com/username)). An issue link alone, or a bare PR number, is not sufficient — e.g.:

- Fixed `/usage` showing Cursor's "Other Models" meter as an inflated dollar amount; it now shows the percentage Cursor reports, like Cursor Models ([#17462](https://github.com/can1357/oh-my-pi/pull/17462) by [@eggpeat](https://github.com/eggpeat))

If the change also tracks an internal issue, keep both links (issue first, then the PR with attribution).

1. User-facing wording: Entries describe the observable behavior change, not the implementation (no internal function/variable names, data-structure details, or root-cause mechanics). Keep the root-cause explanation in the PR description instead. Example: prefer Fixed blob URL index loading blocking startup over a sentence describing the fs.readFileSync-to-async refactor. Avoid overselling impact (e.g., don’t claim a minor allocation reduction makes requests “noticeably faster”).

Before merging, verify the diff touches only ## [Unreleased] in the right package’s CHANGELOG.md, the entry has correct attribution, and the wording is a concise, user-facing summary.