WARDEN-WP-0026 T03: masking display filter (defense-in-depth)

- warden/mask.py: fingerprint()/mask_value() — presence, length, 8-char sha256
  prefix; never the value.
- proxy.proxy_fetch_fingerprint + `warden access --fingerprint`: masked status view
  (presence/length/hash) that emits no value, so it bypasses the T02 stdout guard.
  Lets two parties compare sha256 prefixes to confirm a shared value without seeing
  it (e.g. rotation landed).
- documented as defense-in-depth (raw bao bypasses it) in OperatorAccessAssist.md
  and the module docstring.
- tests: tests/test_mask.py + CLI fingerprint test. 299 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/warden/proxy.py

@@ -301,6 +301,22 @@ def proxy_fetch_wrapped(resolved: ResolvedFetch) -> str:
     return str(token)
 
 
+def proxy_fetch_fingerprint(resolved: ResolvedFetch):
+    """Fetch the value and return a masked fingerprint — never the value (T03).
+
+    Defense-in-depth status view: lets an operator confirm presence/length and
+    compare a short non-reversible hash without disclosing the secret. The value
+    transits warden's memory only to be hashed, and is scrubbed immediately.
+    """
+    from warden.mask import fingerprint
+
+    value = _capture_value(resolved)
+    try:
+        return fingerprint(value)
+    finally:
+        value = ""  # noqa: F841 — best-effort scrub
+
+
 def proxy_exec(resolved: ResolvedFetch, *, env_var: str, child_argv: List[str]) -> int:
     """Fetch the value and inject it into a child command's environment only.