fix(M11): propagate PipelineCriticalError, redact configured secrets, signal blog failures

Correct 4 findings from the independent M11 review:

#1 (Critical) — PipelineCriticalError was downgraded to PipelineWarning:
  - Add except PipelineCriticalError: raise before each except Exception
    in all 5 non-blocking steps (fetch_blog, compare, caldav_sync, synthesis, send)
  - Critical errors now propagate to the outer handler and stop the pipeline

#2 (Critical) — redact_exception() did not use configured secrets:
  - Extend redact_exception() with extra_secrets parameter (upward compatible)
  - Harden redact_secrets(): sort extra_secrets by length descending
  - Add Settings.redaction_secrets() collecting all 6 SecretStr fields
  - Add PipelineRunner._redact(exc) using self._redaction_secrets
  - All except blocks in run() now use self._redact(exc)
  - CalDAV FAILED-status path uses full redaction_secrets collection

#3 (Medium) — BlogRSSClient silently swallowed failures:
  - Add error field to BlogRSSFetchResult
  - rss.py sets error on failure paths (except Exception, bozo/invalid feed)
  - fetch_blog_step raises RuntimeError when result.error is set
  - PipelineRunner now produces PipelineWarning for blog failures

#4 (Medium) — Test coverage at 80%, now 91%:
  - 11 new integration tests covering blog failure/success, compare failure,
    CalDAV failure (exception + FAILED status), send False/exception,
    PipelineCriticalError propagation, secret redaction with sentinel,
    empty agenda/homework, iCal cache cleanup
  - Secret redaction test uses mock (no network) and proves configured-secret
    propagation via non-URL sentinel in RuntimeError

Validation: 619 tests pass, ruff/mypy/bandit/pre-commit green, coverage 91%.

Co-authored-by: opencode/coder <coder@agents.invalid>
Co-authored-by: opencode/test-engineer <test-engineer@agents.invalid>

pronote_sync/pipeline/steps/fetch_blog.py

@@ -24,6 +24,8 @@ def fetch_blog_step(client: BlogRSSClient | None, state: BlogRSSState | None) ->
         result = client.fetch_and_parse(
             known_guids=state.get_known_guids(), etag=etag, last_modified=last_modified
         )
+        if result.error is not None:
+            raise RuntimeError(result.error) from None
         if not result.not_modified:
             state.add_guids(article.id for article in result.articles)
             state.update_cache_headers(result.etag, result.last_modified)