Issue for this PR

Closes #16882

Type of change

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

What does this PR do?

PR #14707 added auto-compaction recovery for 413/context overflow errors, but the ContextOverflowError catch block in processor.ts unconditionally sets needsCompaction = true without checking the compaction.auto config. Users who set compaction: { "auto": false } still get forced into a compaction loop on overflow, and switching to a model with a larger context window doesn't recover the session.

This adds a config.compaction?.auto === false gate to the overflow catch path:

• auto: false — treats the overflow as a normal error: persists it on the assistant message, publishes the error event, sets the session to idle, and returns "stop". The user can then resume with a different model.
• auto: true / undefined (default) — keeps the existing behavior from fix: recover from 413 Request Entity Too Large via auto-compaction #14707 (needsCompaction = true → compact and recover).

The strict === false check means the default (undefined) still compacts, so this is backward-compatible. Manual compaction (/compact) remains available regardless of the auto setting.

How did you verify your code works?

Added two tests in compaction.test.ts:

1. stops on provider overflow when compaction.auto is false — verifies result is "stop", error is persisted as ContextOverflowError, no compaction parts written.
2. compacts on provider overflow when compaction.auto is true — verifies result is "compact", no error on assistant message.

Run with:

cd packages/opencode && bun test test/session/compaction.test.ts
cd packages/opencode && bun typecheck

Also verified no regressions in:

• bun test test/session/retry.test.ts (18 pass)
• bun test test/session/message-v2.test.ts -t "fromError" (6 pass)

Screenshots / recordings

N/A — no UI changes.

Checklist

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