refactor(opencode): cache compaction boundary to skip redundant DB reads

[tobias-weiss-ai-xr]

Fixes #22208

What

filterCompactedEffect reads all messages from DB every loop step, then discards pre-compaction ones. For 500 messages (300 pre-compaction), that's ~10 SQL queries per iteration when only ~4 are needed.

How

• Added since parameter to page()/stream() — adds WHERE clause to skip older messages
• Cache compaction boundary per session (bounded Map, max 1000, FIFO eviction)
• Invalidate cache when new compaction is created

Verification

• bun run typecheck — 19 packages clean
• bun test test/provider/transform.test.ts — 131 pass
• Inspected SQL output to confirm correct WHERE clause with since

[github-actions]

Hey! Your PR title perf: skip re-reading pre-compaction messages from DB every loop step doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.