Run CI on code and the book on prose, not both on everything

[tschm]

PR #17 changed only README.md, docs/, mkdocs.yml and one workflow — and still ran the collector lint and tests, compose parsing, the dashboard checks and CodeQL. None of them can say anything about a markdown file.

Splitting the README into docs/ makes that the common case rather than a one-off, and checks that routinely run over changes they cannot judge are checks people learn to skim past.

The split

The two workflows now divide the repo between them:

	Runs on
ci.yml	everything except **.md, docs/**, mkdocs.yml
book.yml	exactly **.md, docs/**, mkdocs.yml — plus its own path

A mixed pull request still runs both. paths-ignore skips a workflow only when every changed file matches, so a commit touching the collector and its README is fully checked. That is the case that matters, and it is the default.

book.yml lists its own path so a change to the build steps is exercised by the PR that makes it — which is exactly what happens on this PR. Its **.md is deliberately broader than the site itself: it catches collector/README.md and the root README, neither of which is in the book, but a false positive costs one cheap 10-second build while a false negative ships a broken link.

The trap, recorded in ci.yml

main is unprotected today (confirmed: the protection API returns 404), so a skipped workflow costs nothing. If required status checks are ever turned on, a workflow skipped by a path filter never reports at all and the merge button blocks forever. The fix then is a same-named no-op job — not deleting the filter. That is written into the workflow header rather than left to be rediscovered, since this repo grew a branch-protection metric two PRs ago and may well turn protection on.

Verification

• Both files parse, and the resulting trigger blocks are exactly as intended.
• This PR is its own test case: it touches only .github/workflows/*.yml, so CI should run (workflow files are not prose) and Book should run (it lists its own path). If either is missing from the checks below, the filter is wrong.

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