fix(opencode): speed up long session forks

[evan-choi]

Issue for this PR

Closes #41698

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A synthetic 986-message session now forks in 625 ms instead of 4.3 s — 6.8× faster.

Forks publish and project cloned events in 3,000-event batches, while source messages are read in 1,000-message pages. The SQLite schema and single-event publishing path are unchanged.

Benchmark

Environment

• Host: MacBookPro18,1, Apple M1 Pro, 32 GiB, arm64
• Software: macOS 26.5.2, Bun 1.3.14
• Database: Fresh SQLite file per run (MAX_VARIABLE_NUMBER=500000)
• Method: Same worktree and dependencies, 5 runs per revision, session.fork(...) only

Fixture

Messages	Parts	Summary	Durable events	Data
986	5,657 (493 step-finish)	25 MiB	6,644	Generated by the benchmark

Raw results

Run	Baseline 5635afc949 (ms)	PR head 5960d14fd1 (ms)
1	4,274	656
2	4,255	665
3	4,246	613
4	4,081	619
5	4,281	625
Median	4,255	625

Improvement

Median	Reduction	Speedup
4,255 ms → 625 ms	85.3%	6.8×

How did you verify your code works?

• packages/core: typecheck; 1,084 tests passed
• packages/opencode: typecheck; 3,268 tests passed, 22 skipped, 1 todo
• Fork benchmark: 5 runs per revision
• Pre-push workspace typecheck: 30/30 tasks passed

Screenshots / recordings

N/A — this is a non-UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR