feat: add crypto/keyretriever with keychainbreaker integration (#518)

* feat: add crypto/keyretriever package for Chromium master key retrieval

* feat: complete keyretriever with gcoredump, chainbreaker, and tests

* refactor: replace internal chainbreaker with keychainbreaker v0.1.0

Replace the incomplete internal chainbreaker implementation (~1400 lines
of duplicated code) with the external keychainbreaker package, which
provides a complete, well-tested keychain parsing library.

Changes:
- Add github.com/moond4rk/keychainbreaker v0.1.0 dependency
- Update gcoredump_darwin.go to use keychainbreaker API (Open/Unlock/GenericPasswords)
- Add KeychainPasswordRetriever for password-based keychain unlocking
  with sync.Once caching across multiple browser queries
- Unify DefaultRetriever(keychainPassword string) signature across all platforms
- Delete utils/chainbreaker/ (696 lines + test + testdata)
- Delete crypto/keyretriever/chainbreaker_darwin.go (696 lines duplicate)
- Delete browser/exploit/gcoredump/ (duplicate of keyretriever version)
- Update chromium_darwin.go to use keyretriever.DecryptKeychain
- Clean up .golangci.yml lint exceptions and .gitignore entries
- Use errors.Is() instead of == for context.DeadlineExceeded check

* refactor: improve gcoredump exploit code quality and add comments
* fix: address Copilot review feedback on keyretriever

.gitignore

@@ -36,9 +36,6 @@
 # === RFCs ===
 !rfcs/*.md
 
-# === Test fixtures ===
-!utils/chainbreaker/testdata/*.keychain-db
-
 # === Always ignore (override !*/) ===
 .git/
 .idea/