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

crypto/keyretriever/params.go

@@ -0,0 +1,21 @@
+package keyretriever
+
+import (
+	"hash"
+
+	"github.com/moond4rk/hackbrowserdata/crypto"
+)
+
+// pbkdf2Params holds platform-specific PBKDF2 key derivation parameters.
+// Each platform file defines its own params variable.
+type pbkdf2Params struct {
+	salt       []byte
+	iterations int
+	keyLen     int
+	hashFunc   func() hash.Hash
+}
+
+// deriveKey derives an encryption key from a secret using PBKDF2.
+func (p pbkdf2Params) deriveKey(secret []byte) []byte {
+	return crypto.PBKDF2Key(secret, p.salt, p.iterations, p.keyLen, p.hashFunc)
+}