* fix(agents): classify web_fetch error pages as errors, not successful evidence
* fix(agents): classify 502/503/504 error shells as transient, not internal
Per review: a gateway error page is the try-a-different-source case, so
502/503/504 reason phrases now map to transient (warn, then escalate)
while 500/501 stay internal (stop). This mirrors _ERROR_RULES' own split
and removes the two phrases that classified differently through the
shell table than through _classify_error_text ("service temporarily
unavailable", "gateway timeout") — now pinned by a cross-path
consistency test over the live table plus a chain-level test that a 503
page warns and escalates instead of blocking web_fetch on first sight.
Also per review, _classify_error_shell returns a copy of the category
attrs instead of the rules-table dict, matching _classify_error_text.
Dropped "gone" from the phrase table: a single-word reason phrase
collides with legitimate one-word document titles ("# Gone"), and 410
error pages are rare while such titles are not; negative controls record
the decision.
* test(agents): pin crawl4ai's recorded renderer shape; note web_capture asymmetry
Per review: the title rule assumes every web_fetch provider renders
title-first. Eight of the nine providers hold by construction
(f"# {title}" or the shared Article.to_markdown()); crawl4ai renders
server-side, so its generator output (crawl4ai 0.9.2, fit and raw) was
measured over the same corpus and recorded as fixtures driven through
the real tool and middleware chain. A comment on _PAGE_CONTENT_TOOL_NAMES
records that web_capture's absence is intentional: its dead-target signal
is the provider warning suffix, which belongs to the provider boundary.
