[ENG-3666] Add comment and explanation guidance for agents

[devlob]

What this does

Adds a ## Comments and Explanations section to CLAUDE.md, with three rules:

• No ticket IDs anywhere in the tree — not in comments, docblocks, test names, config files or strings. The ID belongs in the PR title and the Closes line, which is where people go looking for the backstory, and git blame gets them there from any line.
• Comments describe the code that is there, not the code that used to be. Narrating the old bug is the PR's job.
• Explanations stay in plain language — short sentences, ordinary words.

The section is byte-identical to the one going into saas and hub, so it diffs cleanly when we amend it.

Because this repo is public, it carries one extra line the others do not: nothing committed here may contain customer names, internal URLs or internal hostnames.

Why

From the team thread: we ship 20–30 PRs a day, and ticket IDs sprinkled through the code make the diffs harder to read while meaning nothing to anyone without Linear open. Dave asked specifically for this repo to be covered, since it is the public one.

Nothing needed removing

git grep -Ini -E '(eng|lab)-[0-9]{3,4}|nightwatch #[0-9]+' over src, tests and field-test returns nothing — the tree was already clean. This is a guideline change only, so the rule keeps it that way.

The ticket ID stays allowed in PR titles here, same as the other repos. About a third of this repo's merged PRs already carry one, and a bare ENG-3568 discloses nothing on its own.

Verification

Check	Result
npm test	1296 passed, 6 skipped (106 files)
npm run typecheck	clean, 8 templates OK

Ref ENG-3666

[coderbuds]

Clear, concise documentation additions improving comment guidelines.

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[devlob]

/review