chore(renovate): simplify quickstart and batch weekly linter updates

[zeitlinger]

Blocked by #253

Summary

• shrink the README quickstart [tools] example to three representative linters
• manage those three README example versions with repo-local Renovate regex managers
• keep the shared default.json free of repo-specific README rules
• update the Renovate drift test so only shared managers are compared against default.json
• keep flint-managed linter updates in a single weekly Monday Renovate PR with a clearer generated title

Why

The previous approach made README quickstart version drift break Renovate PRs, even though the README example is repository-specific. This change keeps the example small, keeps its version bumps automatic, and avoids pushing repo-local behavior into the shared preset.

The follow-up Renovate change also keeps linter bumps batched into one Monday PR, including majors, so the repo gets one coordinated linter-update PR instead of split grouped PRs with generic titles.

Validation

• renovate-config-validator --no-global .github/renovate.json5
• cargo test repo_renovate_config_stays_aligned_with_shared_preset_contract -- --nocapture
• cargo test readme_quickstart_tools_snippets_stay_current -- --nocapture
• cargo test default_renovate_preset_covers_all_linter_tools_weekly
• cargo test repo_renovate_config_stays_aligned_with_shared_preset_contract
• mise run lint:fix

[Copilot]

Pull request overview

Simplifies the README quickstart [tools] example and moves README version maintenance into repo-local Renovate regex managers, while keeping the shared default.json preset contract stable.

Changes:

• Reduce README quickstart tool list to three representative linters (shellcheck, shfmt, actionlint).
• Add repo-local Renovate regex custom managers to update those README example versions automatically.
• Update the Renovate alignment/drift test and committed renovate dependency snapshot to reflect the repo-local README managers.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/registry/tests.rs	Adjusts the Renovate preset alignment test and updates the README quickstart snippet drift test keys.
src/readme_snippets.rs	Shrinks QUICKSTART_KEYS to the three README example tools.
src/bin/sync-readme-snippets.rs	Updates README snippet renderer to output only the three representative tools.
README.md	Shrinks the quickstart [tools] example accordingly.
.github/renovate.json5	Adds repo-local regex managers to update the README quickstart tool versions.
.github/renovate-tracked-deps.json	Tracks the new README regex dependencies in the committed Renovate snapshot.

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