refactor(local-identity): post-Stage4 cleanups and micro-fixes

- audit: chmod only on file creation, not every append (TOCTOU fix)
- jwt_utils: add extract_unverified_payload() helper
- cli: use extract_unverified_payload + JWTError instead of inline decode
- keys: extract _public_key_bytes() helper, import _b64url from jwt_utils
- security: FileNotFoundError try/except instead of path.exists() (TOCTOU fix)
- serve: cache JWK response at server init instead of per-request recompute

138 tests passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

local-identity/src/local_identity/keys.py

@@ -8,7 +8,6 @@ The corresponding public key is never stored separately — it is always
 derived from the private key on load.
 """
 
-import base64
 import hashlib
 import os
 from pathlib import Path
@@ -17,6 +16,7 @@ from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import rsa
 from cryptography.hazmat.primitives.asymmetric.rsa import RSAPrivateKey
 
+from .jwt_utils import _b64url_encode as _b64url
 from .store import _store_dir
 
 
@@ -48,27 +48,28 @@ def ensure_signing_key() -> RSAPrivateKey:
     return private_key
 
 
-def _b64url(data: bytes) -> str:
-    return base64.urlsafe_b64encode(data).rstrip(b"=").decode("ascii")
+def _public_key_bytes(private_key: RSAPrivateKey) -> tuple[bytes, bytes]:
+    """Return (n_bytes, e_bytes) for the public key — shared by key_id and jwk_public."""
+    pub = private_key.public_key().public_numbers()
+    n_bytes = pub.n.to_bytes((pub.n.bit_length() + 7) // 8, byteorder="big")
+    e_bytes = pub.e.to_bytes((pub.e.bit_length() + 7) // 8, byteorder="big")
+    return n_bytes, e_bytes
 
 
 def key_id(private_key: RSAPrivateKey) -> str:
     """Return a stable 16-hex-char key ID derived from the public key modulus."""
-    n = private_key.public_key().public_numbers().n
-    n_bytes = n.to_bytes((n.bit_length() + 7) // 8, byteorder="big")
+    n_bytes, _ = _public_key_bytes(private_key)
     return hashlib.sha256(n_bytes).hexdigest()[:16]
 
 
 def jwk_public(private_key: RSAPrivateKey) -> dict:
     """Return the RSA public key as a JWK dict (RS256, sig use)."""
-    pub = private_key.public_key().public_numbers()
-    n_bytes = pub.n.to_bytes((pub.n.bit_length() + 7) // 8, byteorder="big")
-    e_bytes = pub.e.to_bytes((pub.e.bit_length() + 7) // 8, byteorder="big")
+    n_bytes, e_bytes = _public_key_bytes(private_key)
     return {
         "kty": "RSA",
         "use": "sig",
         "alg": "RS256",
-        "kid": key_id(private_key),
+        "kid": hashlib.sha256(n_bytes).hexdigest()[:16],
         "n": _b64url(n_bytes),
         "e": _b64url(e_bytes),
     }