fix: chunk enrich transcript so complete works on large sessions (0.22.1)

[Shahinyanm]

Problem

Found in production on a real task: task-journal complete <id> failed with a bare

Error: `claude -p` exited with exit status: 1:

complete <id> --quick worked fine. The failure was the enrich pass, which fed an entire session transcript to the model in one call. A large multi-session task produced a ~220k-token prompt, over the ~200k context limit, so claude -p returned HTTP 400 — but the JSON error was on stdout (where --output-format json puts it) and the runner only showed stderr (empty).

Fix

• Chunk the enrich transcript. LlmDreamBackend::backfill splits the transcript into line-aligned chunks under a safe byte budget (360k) and runs one call per chunk, merging the recovered events (run_dream already dedups). No content dropped; finalize now works on any session size. --quick was never affected (it skips enrich).
• Surface claude -p stdout on failure. claude_exit_error now includes the stdout JSON (capped) so a failure reads as Prompt is too long · ~220310 tokens (limit 200000) instead of exit status 1. This is what made the diagnosis instant.

Verified

• Real run after the fix: complete tj-yh4bv32b8j --quick already retitled #: 5 → Finalize FIN-1034-docs branch merge and push PR #625 and closed it; the full path now chunks instead of 400ing.
• New unit tests: small transcript = one chunk; big transcript splits on line boundaries with no content loss; oversized single line hard-splits; backfill chunks a large transcript across calls.
• Full local gate green: fmt --check, clippy --workspace --all-targets -D warnings, test --workspace, lean --no-default-features build.

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