fix: trigger auto-compaction at the configured threshold

[jamestotah]

Issue for this PR

Closes #11314
Refs #15533

Type of change

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

What does this PR do?

This reuses the existing session compaction flow to compact proactively once a completed assistant turn crosses a configurable context threshold.

It adds compaction.threshold, triggers threshold-based auto-compaction in the backend after completed assistant turns, dedupes compaction requests while a session is already compacting, and adds a Server -> Sessions settings surface in the app/Desktop UI.

How did you verify your code works?

• npm exec -- bun test test/session/compaction.test.ts
• npm exec -- tsgo --noEmit in packages/opencode
• npm exec -- tsgo -b in packages/app
• manually set the threshold to 5% in the app and verified a fresh session auto-compacted after crossing it

Screenshots / recordings

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, here are the potentially related PRs (excluding PR #18951, which is the current PR):

Related/Duplicate PRs:

1. PR feat(session): add custom compaction thresholds along with prevention of continuous compaction #10123 - feat(session): add custom compaction thresholds along with prevention of continuous compaction

   • feat(session): add custom compaction thresholds along with prevention of continuous compaction #10123
   • Directly addresses custom compaction thresholds and prevents continuous compaction - very similar scope

2. PR fix(session): prevent infinite loop in auto-compaction when assistant ended its turn #15532 - fix(session): prevent infinite loop in auto-compaction when assistant ended its turn

   • fix(session): prevent infinite loop in auto-compaction when assistant ended its turn #15532
   • Related to the same auto-compaction logic and preventing loops during assistant turns

3. PR feat(compaction): configurable threshold and model (global + per-model) #11931 - feat(compaction): configurable threshold and model (global + per-model)

   • feat(compaction): configurable threshold and model (global + per-model) #11931
   • Addresses configurable compaction thresholds with global and per-model settings

4. PR fix(session): preserve wait-for-user state after compaction #18941 - fix(session): preserve wait-for-user state after compaction

   • fix(session): preserve wait-for-user state after compaction #18941
   • Recently merged PR addressing session compaction state management

These PRs should be reviewed to ensure PR #18951 doesn't duplicate existing work, particularly #10123 and #11931 which explicitly address configurable compaction thresholds.

[jamestotah]

I checked the existing compaction flow before implementing this. This PR does not add a separate compaction mechanism - it reuses the current backend summarize/compaction path and makes it trigger proactively at the configured threshold while deduping in-flight compactions to avoid loops.

[github-actions]

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