fix: stop ignoring content slugs containing cache

[TimeToBuildBob]

Problem

The website's root .gitignore used *cache*, so any new post or asset whose filename contained cache was invisible to normal git status and git add. The content sync could report a successful create while the publish artifact was silently absent from the commit.

This happened while publishing your-cache-can-be-young-and-wrong: the generated _posts/...cache...md file existed, but git status was clean until it was force-added.

Fix

Replace the substring-wide rule with cache-specific directory patterns. Existing build/dependency caches remain ignored, while content slugs containing cache stay trackable.

The brain-side sync now independently fails closed on ignored destinations (ErikBjare/bob@9d78a914c7) so future website ignore regressions are surfaced before the script writes an invisible artifact.

Verification

• .cache/, .*cache*/, *.cache/, *.cache-*, __pycache__/, vendor/, .bundle/, node_modules/, and generated CSS remain ignored.
• _posts/2026-07-15-your-cache-can-be-young-and-wrong.md is not ignored.
• Website pre-commit checks pass.