fix(skills): remove redistribution-restricted built-ins

[bobleer]

Summary

• Stop embedding the redistribution-restricted docx, pdf, pptx, and xlsx skill bundles in future builds.
• Remove those entries from the runtime catalog, mode-policy fixtures, tool examples, upstream manifest, and obsolete Office archive tests.
• Keep ppt-design as the supported built-in presentation workflow.
• Restore the Superpowers MIT notice next to the retained writing-skills files and remove the separately vendored Anthropic authoring guide from that bundle.
• Narrow README and starter copy so it no longer promises the removed built-in export skills.

Upgrade behavior

The embedded skill-tree change produces a new bundle hash. On the next built-in skill sync, BitFun replaces the managed .system directory with the new bundle. The four removed names are also added to first-migration cleanup so historical loose root copies are removed when .system is first created.

This PR only changes future source builds and the normal managed-skill migration path. It does not purge Git history, previous releases, old binaries, or historical release assets. Maintainers should decide separately whether any historical assets need to be removed or rebuilt.

Verification

• pnpm run fmt:rs
• pnpm run i18n:audit
• pnpm run check:repo-hygiene
• git diff HEAD^ --check
• cargo check --locked -p bitfun-core --no-default-features --features agent-runtime
• cargo test --locked -p bitfun-agent-runtime --test agent_definition_contracts skill_contracts:: (29 passed)
• cargo test --locked -p bitfun-core --no-default-features --features agent-runtime --lib agentic::tools::implementations::skills:: (18 passed)