feat(config): declare version guards with required_runok_version

[fohte]

Why

• A preset that depends on newer runok schema features either silently drifts away from the author's intent (older runok ignores unknown fields) or suddenly starts producing parse errors during normal operations
• runok update-presets always jumps to the newest tag regardless of whether it is compatible with the current runok binary, leaving users on older runok without a way to notice that their presets have outgrown their runok

What

• Accept a required_runok_version field in every config and preset file so authors can declare "this file needs runok X or newer" with a semver requirement such as ">=0.3.0" or ">=0.3, <0.5"
  • Every child file reached via extends is validated independently
• Make runok update-presets walk candidate tags from newest to oldest and adopt the newest one whose entire preset tree satisfies the current runok. Rejected candidates are surfaced in a warning so users know that upgrading runok would unlock newer preset versions
• Never update the cache working tree to a revision that is incompatible with the current runok, in both the automatic TTL refresh and the manual update path. This prevents concurrent runok processes from observing a half-updated cache
  • Automatic refresh stays silent and keeps using the existing cache so normal operations do not break
• Treat nightly builds (X.Y.Z-nightly+<sha>) as "latest" so any ">=X.Y.Z" requirement passes automatically

required_runok_version: '>=0.3.0'
definitions:
  flag_groups:
    field-flag: ['-f', '--field']

---

[codecov]

Codecov Report

❌ Patch coverage is 78.85246% with 129 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.13%. Comparing base (02de56c) to head (c4c31ed).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/update_presets/mod.rs	77.10%	49 Missing ⚠️
src/config/git_client.rs	51.94%	37 Missing ⚠️
src/config/preset_remote.rs	85.26%	28 Missing ⚠️
src/config/preset.rs	45.00%	11 Missing ⚠️
src/config/cache.rs	81.25%	3 Missing ⚠️
src/config/loader.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
- Coverage   89.55%   89.13%   -0.43%     
==========================================
  Files          52       53       +1     
  Lines       11015    11538     +523     
==========================================
+ Hits         9865    10284     +419     
- Misses       1150     1254     +104     

Flag	Coverage Δ
Linux	88.94% <78.85%> (-0.42%)	⬇️
macOS	90.37% <78.85%> (-0.52%)	⬇️

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