chore(root): drop three redundant top-level files (#7839)

* chore(root): drop redundant top-level files

Three small clean-ups to make the project root easier to scan for new
contributors:

- best_practices.md was a near-duplicate of CONTRIBUTING.md. Merge its
  two unique bullets (PRs MUST include a description / flag empty
  descriptions as incomplete) into CONTRIBUTING.md and remove the file.
- .pr_agent.toml is a two-line Qodo PR-bot config. It has no functional
  references in the repo and Qodo currently doesn't review this repo.
- tests/ -> src/tests was an unused root symlink. Verified no workflow,
  script, eslint config, or playwright config relies on the root path
  (all callers use src/tests/... or the ep_etherpad-lite package path).

No CI, build, or docs updates needed.

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

* chore(root): fix dead tests/frontend + best_practices.md refs

Follow-up on Qodo review feedback for #7839:

- CONTRIBUTING.md: front-end-tests path now reads src/tests/frontend/
  instead of the dead tests/frontend/ (the root tests symlink is gone).
  The browser URL <yourdomainhere>/tests/frontend stays — that's an
  Express route served by the test runner, not a filesystem path.
- docs/superpowers/specs/...openapi-design.md: drop best_practices.md
  from the parenthetical list of policy sources (now CONTRIBUTING.md
  and AGENTS.MD), since best_practices.md has been removed.

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

---------

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

Author: JohnMcLear

.pr_agent.toml

@@ -5,6 +5,8 @@
 
 ## Pull requests
 
+* PRs MUST include a non-empty description explaining what the change does and why
+* PRs without a description should be flagged as incomplete
 * the commit series in the PR should be _linear_ (it **should not contain merge commits**). This is necessary because we want to be able to [bisect](https://en.wikipedia.org/wiki/Bisection_(software_engineering)) bugs easily. Rewrite history/perform a rebase if necessary
 * PRs should be issued against the **develop** branch: we never pull directly into **master**
 * PRs **should not have conflicts** with develop. If there are, please resolve them rebasing and force-pushing
@@ -123,7 +125,7 @@ Documentation should be kept up-to-date. This means, whenever you add a new API
 You can build the docs e.g. produce html, using `make docs`. At some point in the future we will provide an online documentation. The current documentation in the github wiki should always reflect the state of `master` (!), since there are no docs in master, yet.
 
 ## Testing
-Front-end tests are found in the `tests/frontend/` folder in the repository. Run them by pointing your browser to `<yourdomainhere>/tests/frontend`.
+Front-end tests are found in the `src/tests/frontend/` folder in the repository. Run them by pointing your browser to `<yourdomainhere>/tests/frontend`.
 
 Back-end tests can be run from the `src` directory, via `npm test`.
 You can use `npm test -- --inspect-brk` and navigate to `edge://inspect` or `chrome://inspect` to debug the tests.

CONTRIBUTING.md

@@ -197,7 +197,7 @@ GET /admin/openapi.json   (CORS: *)
 
 The route is gated by `settings.adminOpenAPI.enabled`, **default `false`**,
 per the project's "new features behind a flag, off by default" policy
-(CONTRIBUTING.md, AGENTS.MD, best_practices.md). When the flag is off,
+(CONTRIBUTING.md, AGENTS.MD). When the flag is off,
 `expressPreSession` returns early and the route is dormant.
 
 When enabled, the route registers in `expressPreSession`, which runs before