refactor: extract --fix execution logic from src/index.ts to src/utils/fix-runner.ts

[sonukapoor]

`src/index.ts` contains roughly 180 lines of `--fix` execution logic that has no reason to live in the entry point:

• `FixExecutionResult` type (line 497)
• `applyFixesIfRequested()` (line 378) — orchestrates spinner, runs install commands per target, returns a result object
• `buildFixCommandParts()` (line 451) — builds the install command array for a given package manager
• `runInstallCommand()` (line 467) — spawns the child process and resolves on exit
• `commandLabelForPackageManager()` (line 490) — maps package manager to display label
• `countBySeverity()` (line 506) — counts findings by severity label
• `printFixModeSummary()` (line 521) — prints the applied/skipped summary after `--fix` completes

Work to do:

1. Create `src/utils/fix-runner.ts`
2. Move all the above and export what `src/index.ts` needs
3. Update `src/index.ts` to import from `./utils/fix-runner.js`
4. Run `npm test && npm run build` to confirm nothing broke

No behaviour changes — pure extraction.