docs: delete executed design specs and check every doc link

[ttncode]

What this changes

• Deletes docs/superpowers/ (8 specs, 2 plans). All were executed; the decisions they carried live in docs/decisions/, the history in git.
• Drops the two references to them (docs/PROVENANCE.md, docs/tour/09-wizard.md), keeping the facts inline.
• tests/documentation.bats now checks backticked paths in README.md, CONTRIBUTING.md and docs/README.md too, and checks that every relative link and image under README.md, CONTRIBUTING.md and docs/ resolves.
• CONTRIBUTING.md: you/.github is a repository name, not a path in this repo, so it is no longer backticked.

How it was verified

• bats tests/documentation.bats: widened path test failed with missing: you/.github before the fix, 5/5 pass after.
• mise run test-unit: 199/199. mise run lint: clean.

Checklist

• mise run lint passes
• mise run test-runner passes — docs and a doc test only; CI runs it
• New behaviour has a test that fails without the change
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes