feat(agent): cak_ at-rest credential store (SPEC-016 Phase B item 4)

Store the per-machine cak_ with BOTH layers Mike locked: DPAPI-machine
encryption (CryptProtectData with CRYPTPROTECT_LOCAL_MACHINE — a copied
blob is inert off the box) inside a SYSTEM/Administrators-only ACL'd file
at %ProgramData%\GuruConnect\credentials\agent.cak. The directory + file
ACL is hardened via icacls (/inheritance:r + grant to the well-known SIDs
*S-1-5-18 and *S-1-5-32-544, locale-independent) — auditable, with far
less unsafe FFI than building a registry-key security descriptor by hand.
Co-locates with the existing %ProgramData%\GuruConnect config/seed dir.

Provides store_cak / load_cak / clear_cak. store_cak writes atomically
(temp file + rename in the locked dir). load_cak treats a present-but-
undecryptable blob as a hard error (tamper / cross-machine copy) rather
than silently re-enrolling over it. The plaintext is never logged; the
transient plaintext copy is scrubbed after encryption. DPAPI output blobs
are LocalFree'd. Enables the Win32_Security_Cryptography windows feature.

Round-trip unit tests cover encrypt/decrypt recovery across lengths and
that a tampered blob fails to decrypt (DPAPI authenticates its blobs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/Cargo.toml

@@ -92,6 +92,7 @@ windows = { version = "0.58", features = [
     "Win32_System_Console",
     "Win32_System_Environment",
     "Win32_Security",
+    "Win32_Security_Cryptography",
     "Win32_Storage_FileSystem",
     "Win32_System_Pipes",
     "Win32_System_SystemServices",