docs: fix broken nix command and stale references in contributor docs

[prql-bot]

Four stale or incorrect references in the contributor docs, each checked against the code or run in a shell.

web/book/src/project/contributing/development.md

The nix setup command doesn't work. Step 2 of "Option 4: Use nix development environment" says tee 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf. tee's argument is a filename, not content — so this creates a file literally named experimental-features = nix-command flakes in the current directory, blocks reading stdin from the terminal, and leaves nix.conf empty. Anyone following the nix path today gets no flakes and a junk file. Changed to echo.

Python version is two releases behind. The doc says "ensure we have Python >= 3.7"; prqlc-python/pyproject.toml declares requires-python = ">=3.9".

"GitHub Actions on merge" points at a deleted workflow. The link was SHA-pinned to c042eef4's .github/workflows/pull-request.yaml#L124; that file no longer exists on main. Push-to-main test runs are now tests.yaml, which declares push: branches: [main] and gates the wider matrix through the rules job's main output — so the surrounding prose still describes real behavior, just via a different file. Repointed to tests.yaml. Left unpinned deliberately: it's a whole-file reference with no line anchor, and the intent is "wherever this lives now".

CLAUDE.md

Integration test path is missing a directory level. It reads prqlc/tests/integration/queries/*.prql; the actual location is prqlc/prqlc/tests/integration/queries/. prqlc/tests/ doesn't exist. Same class of slip as #6108, one directory up.

The doc-viewing snippet invokes a View tool that doesn't exist. Claude Code's file-reading tool is Read; there is no View. Following the snippet as written fails.

Verification

The tee behavior, run in a scratch directory:

$ cd /tmp/teetest && tee 'experimental-features = nix-command flakes' >> ./nix.conf < /dev/null
$ ls -la
-rw-rw-r-- 1 ... 0 ... 'experimental-features = nix-command flakes'
-rw-rw-r-- 1 ... 0 ... nix.conf

Both files are 0 bytes — the setting never reaches nix.conf. (< /dev/null stands in for the terminal here; interactively it would hang instead.)

Paths:

$ test -d prqlc/tests/integration/queries        # absent
$ test -d prqlc/prqlc/tests/integration/queries  # present
$ ls .github/workflows/ | grep pull-request
pull-request-target.yaml

requires-python is read directly from pyproject.toml (linked above).

No regression test. These are all prose and link fixes with no executable surface; the shell transcripts above are the check. The one testable claim — the missing directory level in CLAUDE.md — is verified by the test -d pair.