docs(security): correct bcrypt cost + remove false encryption-at-rest claim

Two doc errors caught by the 2026-05 security audit (P0-3):

1. SECURITY.md claimed bcrypt cost 10. The code has been using cost 12
   since gearbox/internal/framework/auth/password.go was written; the
   doc just never caught up. Cost 12 is the 2026 OWASP baseline, so
   the code is right and the doc was lagging.

2. SECURITY.md claimed "Credential Encryption: API keys encrypted at
   rest (AES-256-GCM)". That implementation does not exist anywhere in
   the codebase — a grep for "aes" / "GCM" / "cipher" returns zero hits
   in gearbox-agent. Secrets are protected by filesystem permissions
   (mode 0600) only. The claim is replaced with an honest description
   of the actual posture, plus guidance on how to layer real
   encryption-at-rest underneath (FDE / KMS).

Also adds the 2026-05 audit to the Security Audit History table.

The "real fix" for #2 (implementing envelope encryption with a KMS- or
TPM-backed key) is tracked separately; co-locating the decryption key
with the ciphertext on the same filesystem buys nothing real and the
honest doc is more defensible than the theatrical implementation.

Author: sarg3nt

SECURITY.md

@@ -147,7 +147,7 @@ Gearbox includes the following security features:
 
 ### Authentication & Authorization
 
-- **Bcrypt Password Hashing**: Industry-standard password hashing (cost 10)
+- **Bcrypt Password Hashing**: Industry-standard password hashing (cost 12; OWASP 2026 baseline)
 - **Session Management**: Secure, HTTP-only session cookies
 - **Password Requirements**: Minimum 50 bits entropy, blocks common passwords
 - **Multi-factor Support**: WebAuthn/Passkey support for strong authentication
@@ -171,7 +171,13 @@ Gearbox includes the following security features:
 ### Data Protection
 
 - **Secret Redaction**: Automatic redaction in debug logs
-- **Credential Encryption**: API keys encrypted at rest (AES-256-GCM)
+- **Filesystem-Protected Secrets**: API keys, webhook secrets, and admin
+  credentials are stored as files owned by the agent user with mode `0600`
+  (owner read/write only). Encryption-at-rest is **not** currently
+  implemented; if the filesystem is compromised by an attacker with the
+  agent's UID or root, secrets are exposed. Deployments that need
+  encryption-at-rest should run the agent on a filesystem with FDE
+  (LUKS, FileVault, etc.) or use an external KMS-backed secret store.
 - **Secure Password Storage**: Admin credentials written to file with 0600 permissions
 - **Session Secrets**: 256-bit minimum session secret requirement
 
@@ -195,9 +201,10 @@ Subscribe to the repository to receive notifications of security updates.
 
 ## Security Audit History
 
-| Date       | Auditor        | Scope          | Findings | Status    |
-|------------|----------------|----------------|----------|-----------|
-| 2026-01-31 | Internal Scan  | Full codebase  | 10 total | ✅ Resolved |
+| Date       | Auditor               | Scope                                       | Findings                        | Status         |
+|------------|-----------------------|---------------------------------------------|---------------------------------|----------------|
+| 2026-01-31 | Internal Scan         | Full codebase                               | 10 total                        | ✅ Resolved     |
+| 2026-05-10 | Internal Deep Audit   | Auth, agent API, HAProxy pipeline, SQL/XSS, agent subprocess | 4 P0, 8 P1, 10 P2, 8 P3 | 🔄 In progress |
 
 ## Contact