feat: research that finishes, statutes as sources, and freer document editing

Three pieces of work that were running live but had never been committed.
They share several files, so they could not be separated into commits that
each still build.

Research that finishes. A chat that searched, found law and then stopped
with no answer was hitting a hidden limit of ten rounds of tool use; short
lookups fit inside it and real research did not. There is now one shared
budget across every model, and running out of it never ends a turn in
silence - the assistant is made to answer from what it has and say what it
could not check. Where it stops, a Keep going button carries on from that
point instead of starting again.

Statutes as sources. A statute the assistant looks up now appears as a
numbered source that opens in the reading panel, the same way cases do,
rather than as loose text in the reply.

Freer document editing. Edits to a document can now add and remove whole
paragraphs instead of only changing words already there, which is what
writing a letter on the firm letterhead needs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PoCRDAPyEq4wwBVFFbDzck