fix(storage): survive unreadable artifact files and pin utf-8 i/o

[plind-junior]

What changed

makes bulk artifact listing resilient to a single unreadable file, and pins
encoding="utf-8" on every text-mode file i/o under src/vouch/.

Why

vouch pending — and every bulk list_* path — crashed with a yaml
ReaderError when one proposal file held a control character: a single bad
file took down the whole listing. root cause: vouch's read_text /
write_text / open relied on the locale default encoding, so on a non-utf-8
locale (e.g. latin-1) non-ascii claim text was mangled into raw bytes the yaml
loader rejects — and writes could crash outright. the fix has two layers:
skip-and-warn on an unreadable file (defense in depth) plus pin utf-8
everywhere (the actual root cause).

What might break

nothing for existing .vouch/ users on a utf-8 system. on a non-utf-8 locale,
new writes are now correct utf-8; a file already corrupted by the old
locale-encoded write path is now skipped with a warning instead of crashing the
command (re-save it as utf-8 to recover it).

VEP

not a surface change — no VEP needed.

Tests

• ruff + mypy clean; pytest green except the pre-existing
  test_example_screenshots svg-render check (unrelated, font-metric
  environmental — fails on main/test too)
• new regression tests: a corrupt proposal/claim file is skipped and the
  listing still returns the good artifacts
• CHANGELOG.md updated under ## [Unreleased] → ### Fixed

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