fix(verify): wrong exception type caught — verification crashes on missing content

[aliangm]

verify_source() catches FileNotFoundError but store.read_source_content() raises ArtifactNotFoundError (a KeyError subclass) when content is missing. The mismatch means a single source with missing content crashes the entire verification sweep.

Bug

• File: src/vouch/verify.py:30-31
• Buggy code:

def verify_source(store: KBStore, source: Source) -> VerificationResult:
    try:
        body = store.read_source_content(source.id)
    except FileNotFoundError:  # <-- wrong exception type
        return VerificationResult(source=source, stored_ok=False,
                                  external_status="n/a", note="stored content missing")

• Correct pattern (from storage.py:217-221):

def read_source_content(self, source_id: str) -> bytes:
    p = self._source_dir(source_id) / "content"
    if not p.exists():
        raise ArtifactNotFoundError(f"source content {source_id}")  # KeyError subclass
    return p.read_bytes()

Steps to Reproduce

1. vouch init && vouch source add some-file.txt (registers a source)
2. Manually delete .vouch/sources/<sha256>/content
3. Run vouch source verify or vouch doctor
4. Result: unhandled ArtifactNotFoundError traceback instead of graceful "stored content missing" report

Expected Behavior

verify_source should catch ArtifactNotFoundError and return a VerificationResult with stored_ok=False, as the code clearly intended.