fix(skills): an unreadable skills dir must not abort the run (OPE-216)

[devikaverma]

Two bugs found together while evaluating against benchmark datasets, with OpenWorker running
headless as an unprivileged user. The second made the first hard to find.

What

SkillLoader._discover made its filesystem calls unguarded, and that scan is on the per-turn
path — agent.py:663 calls rescan() every turn to rebuild the live skill menu. pathlib
ignores ENOENT/ENOTDIR/EBADF/ELOOP but propagates EACCES, so an unreadable
<workspace>/.coworker/skills — optional and usually absent — raised straight out of the turn
and ended the run. At startup the directory does not exist, ENOENT is ignored and the session
begins normally, which is why it struck at an arbitrary turn rather than at launch; long runs
died around 100 turns deep, after the work was already done.

The failure was then misreported. _NO_ACCESS listed "permission denied", matched against
str(exc).lower(), and every OS PermissionError stringifies as [Errno 13] Permission denied: <path> — so an unreadable file anywhere in a turn produced "Your account doesn't have
access to <model>", a billing problem that did not exist, sending debugging after a provider
outage that never happened.

• skills/base.py: _discover degrades to "no skills here" on OSError, and each skill's
  parse is guarded separately so one unreadable or malformed folder costs only that skill
  rather than the rest of the directory.
• providers/errors.py: drop the "permission denied" marker. Anthropic's genuine marker is
  the underscored type permission_error, already listed and still matching; the OpenAI
  markers are untouched. Unrecognised errors return None so the caller surfaces the raw
  message.
• Tests: two for the skills scan (unreadable dir, unreadable single folder), one for the error
  classifier.

Testing

• tests/test_skills.py and tests/test_model_errors.py: 14 passed (3 new). All three new
  tests fail against unmodified main.
• Skills and provider-error suites: 86 passed, 1 skipped.
• Full suite on Windows: 2627 passed; the 45 failures are pre-existing and identical on
  unmodified main (toolchain, tool-request, ui-refresh, gallery fixtures).
• A/B in a container with the trigger manufactured (mkdir /src/.coworker && chmod 000): the
  unfixed build crashed and misreported it, the fixed build returned skills=[] and classified
  the error correctly.