Add safe rules for pnpm and Yarn package caches

[Muawiya-contact]

Goal

Add rules for common pnpm and Yarn cache folders.

Diskern already has npm cache coverage. pnpm and Yarn users should get the same safe, clear classification where the cache is regenerated by the package manager.

Suggested starting point

Rules live here:

crates/diskern-core/rules/base.json

Existing npm cache rule can be used as the model:

{
  "id": "npm-cache",
  "patterns": ["**/.npm/_cacache/**"],
  "category": "package_manager_cache",
  "verdict": "safe",
  "description": "npm's download cache. Re-fetched on demand; removing it only makes the next install download packages again."
}

Please research the common pnpm/Yarn cache paths and add conservative rules only.

Acceptance criteria

• Adds pnpm and/or Yarn cache rules.
• Uses package_manager_cache and a safe/review verdict only when the path is clearly cache.
• Adds or updates tests in crates/diskern-core/src/rules.rs.

[LouisDeconinck]

I'll take this — adding conservative package_manager_cache rules for the pnpm store and Yarn caches (Yarn 1 global cache + Berry per-project/global caches), modeled on the npm-cache rule, with fixture rows in EMBEDDED_RULE_FIXTURES and tests that the surrounding non-cache paths stay unknown.

[Muawiya-contact]

@LouisDeconinck have to read the discussion? I leave a message for you.
#149