fix(doc-gardening): remove git operations from lint path

[felladrin]

Problem

npm run lint runs scripts/doc-gardening.cjs, which unconditionally calls createFixupPRs(). That function:

1. git checkout -b doc-gardening-<timestamp> — switches your branch, carrying your working tree onto a new branch.
2. applyFix — rewrites offending docs, replacing broken links with [REMOVED BROKEN LINK].
3. git add . — stages everything in the tree, including unrelated work.
4. git commit -m "docs: fix documentation issues found by doc gardening" — commits it all.

Despite the name, it never opens a PR; it prints the git push and gh pr create commands for you to run manually.

Solution

• garden() no longer calls createFixupPRs() unconditionally.
• The automated fix-up is now behind an explicit --fix flag.
• --fix also requires a clean working tree (checked via git status --porcelain) so it can never sweep unrelated changes into a commit.
• Broken relative links now fail lint with exit code 1 and a message naming the file and target.
• The freshness check already reports stale docs; no change needed there.

Acceptance criteria

• npm run lint never runs git checkout, git add or git commit.
• A broken relative link in docs/ fails lint with a message naming the file and the target.
• The freshness check reports stale docs (already worked; body was not empty).