fix(session): cap per-message summary diff size to prevent OOM bloat

[YOMXXX]

Summary

Fixes #1854 (part of it). message.summary.diffs grow unbounded — single messages reach tens–hundreds of MB because Snapshot.FileDiff.patch holds the full file content (diffFull uses full context). On resume, JSON → V8 heap ×copies → 10–20 GB process commit → OOM / Windows Event 2004.

Root cause: packages/opencode/src/session/summary.ts summarize() stores the full per-message diff into the message summary with no size cap.

Fix: cap the per-message summary diff at MAX_SUMMARY_DIFF_BYTES (1 MB):

• capSummaryDiffs() slices each FileDiff.patch to the remaining byte budget and drops entries once exhausted, so a single huge patch can never bloat a message beyond ~1 MB.
• Applied only to the message-embedded copy. The full session diff (session_diff) and the revert path are untouched — they use the unbounded diff.

Test Plan

• New tests: huge patch capped to budget; small diffs unchanged; budget exhaustion drops entries.
• bun test test/session/summary-diffs-cap.test.ts — 3 pass; full session suite 913 pass no regression.
• bun typecheck — clean.

Notes

• Tradeoff: the messageID-scoped diff API returns truncated (structurally-invalid) diffs for huge single-file edits. No in-repo consumer uses the messageID variant (the TUI uses the session-level diff); the cap solves the OOM at the cost of per-message diff fidelity for pathological edits.
• The other Process OOM 10–20GB commit: unbounded message.summary.diffs + prune skipped while checkpoint-writer + history.backfill (related #1351) #1854 acceptance items (prune deferral, history.backfill, process memory budget, fork slim-history, metrics) are separate fixes; this addresses the single largest lever (unbounded per-message diff JSON).