Review fixes: opaque page_token cursor, raw-next has_more, FAQ citation

Round-1 review blocker: the path-shaped page_token with a
startsWith("/matters.json") guard was traversal-bypassable because
buildUrl's new URL() normalises ".." segments (DURABLE_LESSONS
2026-07-28 class). Redesigned to an opaque cursor:
matterPageTokenFromNext now parses meta.paging.next, requires the exact
Manage API origin and exact <apiBase>/matters.json pathname, and
extracts only the page_token query-param value; the continuation
request is rebuilt from named parts (hardcoded path, our fields+limit,
repeated query/status per the schema) so a model-supplied string never
becomes a request path. has_more now derives from the raw presence of
meta.paging.next, never from cursor extraction success. Status-filter
citation pinned to docs.developers.clio.com/faq/ (case sensitivity
undocumented; lowercase literals enforced); at-least-one-argument rule
stated in the function description. Tests: traversal/suffix-path/
off-host/protocol-relative next URLs rejected, traversal-shaped
page_token asserted to never reach the path, continuation rebuild
asserted to carry fields+limit+cursor. 620/620 tests, tsc + prettier
clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>