Journal-backed history follow-ups (#854): remaining work after #3852 #3868

PR #3852 moves conversation history onto an on-disk journal and streams OpenAI chat, OpenAI Responses and Anthropic request bodies from it. This issue tracks the work that did not go into that PR, plus the fixes #3852 itself closes, so all of the #854 follow-up lives in one place.

Closed by #3852

• OpenAI/Responses requests now upload chunked without Content-Length (on-demand body from #854) #3847 Content-Length restored on streamed request bodies
• Last-resort tool-response truncation is O(candidates x rows) on the journal-backed send path #3857 last-resort truncation is quadratic and keeps a disk copy per candidate
• Responses progressive input materializes the whole history when a tool row is present #3858 Responses progressive input materializes the whole history
• Shared-history recorder fan-out: unowned rewinds reach every mirror and can over-remove; resumed recorder becomes unfiltered primary #3865 shared-history fan-out mirrors drop writes and filter origins wrongly
• Request-artifact telemetry: fixed 5 s export acknowledgement fails slow-but-healthy writes and leaves abandoned writes running #3866 telemetry export acknowledgement fails slow-but-healthy writes
• HistoryJournalStore.dispose() is fire-and-forget: recorder dispose errors are swallowed and scratch is removed asynchronously #3867 HistoryJournalStore.dispose() swallows errors

Deferred

• Unified scrollback to the start of history (Unified scrollback to the start of history: one disk-paged surface with the live turn, on by default #3856), then the in-context marking and summary expansion UI that depends on it. These are the remaining Cache and load conversation history and highlight what is in context #854 UI asks; Cache and load conversation history and highlight what is in context #854 stays open until they land.
• Gemini plugin and Vercel providers still build the whole history as an array per send (declared exception, SDK-bound).
• Incremental provider normalization (Incremental provider normalization across sends (journal-backed history send cost grows with row count) #3849), to be re-measured after feat(history): file-backed conversation history streamed to providers (Fixes #854) #3852's streamed bodies.
• Array-route leftovers and heap harness cleanup (Remove array-route leftovers and #854 heap harnesses after the journal-fed send route #3848, remaining part).
• Test coverage follow-ups from the feat(history): file-backed conversation history streamed to providers (Fixes #854) #3852 review.
• Provider package boundary: providers depend on history internals.
• Record what a history publication means instead of inferring it from row shapes.

Checks after #3852 merges

• Re-verify HistoryService: direct mutations bypass the mutation FIFO and can drop a concurrent add #3443 and AgentClient.dispose() gates media release on stale _previousHistory, leaking reservations from ordinary turns #3445 against the rewritten history code; close with evidence or adopt them here.
• Core's export map grew from 171 to 208 entries on the feat(history): file-backed conversation history streamed to providers (Fixes #854) #3852 branch; see Finish package boundary enforcement: trim 117-subpath core exports, remove tsconfig bypasses, break core-mcp cycle (Fixes #2618) #2618.