refactor: add adaptive fast-only run policy

[zeitlinger]

Summary

• split init grouping from --fast-only behavior by introducing a separate run policy
• mark renovate-deps as adaptive and only run it in --fast-only when the change set is relevant
• add unit and e2e coverage for the renovate relevance gate, and ignore the local .cache/ dir

Testing

• mise exec -- cargo test -q
• mise run lint:fix

[zeitlinger]

Added a follow-up commit on this stack:

• switch biome to the standalone mise package instead of the npm backend
• replace markdownlint-cli2 + prettier with rumdl for Markdown and yaml-lint for YAML
• update flint init scaffolding to generate .rumdl.toml / .yamllint.yml and keep workflow-driven YAML selection idempotent
• migrate the repo’s own mise.toml, docs, knowledge notes, and fixture coverage to the new stack
• refresh the Renovate tracked dependency snapshot for the new tool keys

Verification I ran locally on this branch:

• cargo test -q readme_linter_table_in_sync
• FLINT_CASES=general cargo test -q cases
• FLINT_CASES=rumdl cargo test -q cases
• FLINT_CASES=yaml-lint cargo test -q cases
• cargo test -q markdown_tool_ignores_biome_owned_jsonc
• cargo run -q -- run --fix with a local PATH wired to the repo-local tool installs

I could not use a normal mise exec -- cargo test -q in this environment because rustup-backed mise installs want to write under read-only locations unless redirected.

[zeitlinger]

Follow-up note for the docs/config migration: yamllint-rs currently publishes only Linux release binaries (yamllint-rs-linux-amd64 and yamllint-rs-linux-arm64), so flint is using cargo:yaml-lint for now. Upstream follow-up: open a PR / issue on AvnerCohen/yamllint-rs to publish macOS and Windows binaries too, ideally with a stable asset naming story that can map cleanly to the yaml-lint command. If that lands, we can reconsider switching this check from cargo: to a GitHub-release-backed install path, similar to the other standalone tools we use.