Fix cache path inconsistency when running from non-project directory

[Copilot]

CacheService defaulted its cache directory to Directory.GetCurrentDirectory()/.cache, while --clear-cache resolved the path via repoRoot. Running from the repo root (e.g., dotnet run --project src/NewsletterGenerator) caused cache clears and cache reads/writes to target different directories.

Changes

• RunGenerateAsync: Compute cacheDir from repoRoot once, reuse it for both the clear-cache logic and the CacheService constructor — replacing the implicit cwd-based default.

// Before: cacheDir scoped inside clearCache block; CacheService used cwd implicitly
var cache = new CacheService(loggerFactory.CreateLogger<CacheService>(), forceRefresh: forceRefresh);

// After: single canonical path passed to both
var cacheDir = Path.Combine(repoRoot, "src", "NewsletterGenerator", ".cache");
// ...clearCache logic uses cacheDir...
var cache = new CacheService(loggerFactory.CreateLogger<CacheService>(), cacheDir, forceRefresh: forceRefresh);

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