Compaction: Configurable preserved turns and prune thresholds

[riatzukiza]

Epic: #46

Summary

Expose key compaction knobs in config for tuning and experiments.

Proposal

• experimental.chatPreservedTurns (default 6)
• experimental.pruneProtect and experimental.pruneMinimum
• experimental.compactionMode = "handoff" | "lite"

Code Refs

• packages/opencode/src/session/compaction.ts:43-45 (PRUNE_* constants)
• packages/opencode/src/session/compaction.ts:263 (chatMaxRetries example)
• packages/opencode/src/config/config.ts (Config.get)

Tasks

• Add experimental config options and wire through call sites
• Document in README or config schema

Acceptance Criteria

• Users can tweak preserved turns and prune thresholds without code changes.

[riatzukiza]

Related Issues & Context

This Configurable Options issue enables fine-tuning of ALL other compaction improvements:

Direct Configuration:

• Compaction: Smarter boundary selection and cross-session guard #47 - Boundary Selection: Implements (default 6) - core boundary logic
• Compaction: Pruning/soft compaction and selective tool-output deletion #50 - Pruning: Implements and thresholds
• Compaction: Structured BUILD-mode handoff summary prompt #49 - Structured Handoff: Implements for "handoff" vs "lite" modes

User Control:

• Compaction: Telemetry and /status for context + decisions #52 - Telemetry: Users can tune thresholds based on telemetry insights
• Compaction: Provider options flow and accurate usage accounting #51 - Usage Accounting: Config helps optimize cost vs performance tradeoffs
• Epic: Compaction Update #46 - Compaction Update Epic: Configuration is key to epic's optimization goals

Strategic Importance:

• Epic: Subagent State Handoff & Context Boundary Resilience #56 - Subagent State Handoff: The configuration patterns established here extend to subagent settings:

Implementation Dependencies:

• Compaction: Tests – boundary, rules, pruning, integration #54 - Tests: Configuration options need comprehensive test coverage for edge cases
• Compaction: Concurrency/locking robustness and retries #53 - Concurrency: Configuration changes must be safe under concurrent operations

User Experience:

Configurable options are CRITICAL for:

• Power users fine-tuning for specific workflows
• Development experimentation with different strategies
• Performance optimization for different hardware/contexts
• Cost management for budget-constrained users

Experimental Strategy:

The namespace allows:

• Safe rollout of new features
• A/B testing of different strategies
• Gradual migration from current behavior
• User feedback collection before stabilization

Cross-Cutting Impact:

Configuration framework developed here serves BOTH:

1. Compaction tuning - optimizing context transfer within sessions
2. Subagent tuning - optimizing context transfer between agents

Future Extensibility:

The configuration infrastructure established here becomes the template for:

• Subagent behavior tuning
• Context boundary policies
• Performance optimization settings
• User preference management

Without proper configurability, both compaction and subagent features remain one-size-fits-all solutions that can't adapt to different use cases.