deps: consolidate Renovate package updates

[gjbkz]

Upgrade commander 11.1.0 → 15.0.0, AVA 5.3.1 → 8.0.1, and both typescript-eslint packages 6.21.0 → 8.71.1 together, replacing the updates proposed in #701, #697, #692, and #690. Regenerate package-lock.json with the combined dependency graph.

Commander 15 requires Node >=22.12.0, so update the package engine and README accordingly. Migrate the shared ESLint configuration's removed typescript-eslint rules to their v8 replacements, use nullish defaults for commit subjects, and simplify the record-like guard to satisfy the updated rules.

Stacked on #714, which is stacked on #713. Merge #713 → #714 → this PR (#709 is already merged). GitHub Actions dependency updates remain separate.

Validation: clean offline npm ci; TypeScript build; 47 tests passing on Node 24 and 26; lint and formatting checks; four CLI smoke checks on the minimum supported Node 22.12.0; npm pack confirms source maps and test files are excluded.

PR CI: all 10 jobs passed across Linux, macOS, and Windows on Node 22/24/26. #722 is stacked next to authenticate Codecov uploads.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.00%. Comparing base (b764246) to head (9dd2616).
⚠️ Report is 138 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cli.mts	0.00%	1 Missing ⚠️
src/getCommitMessageFile.mts	0.00%	1 Missing ⚠️
src/parseMessage.mts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           main     #720       +/-   ##
=========================================
+ Coverage      0   95.00%   +95.00%     
=========================================
  Files         0        8        +8     
  Lines         0      260      +260     
  Branches      0       42       +42     
=========================================
+ Hits          0      247      +247     
- Misses        0       12       +12     
- Partials      0        1        +1     

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