fix: ignore nested snapshot directories in size cap

[C1-BA-B1-F3]

Summary

• Apply snapshot-copy ignore rules at any path depth for size estimation, copy filtering, and post-copy pruning.
• Replace the raw du size estimate with a directory walk that skips ignored directories.
• Add regression coverage for nested node_modules and dist directories.

Closes #262

Tests

• npx tsc -p tsconfig.json --noEmit
• node --import tsx --test tests/always-on/workspace/SnapshotCopyProvider.test.ts
• npm test