feat(compaction): add configurable protectedTools to exempt tools from pruning

[marlian]

Summary

• Adds compaction.protectedTools config option (string[]) to specify additional tool names that should be exempt from context pruning
• The built-in "skill" tool remains always protected as a hardcoded default
• User-specified tools are merged with defaults using concat + dedup, following the existing pattern for plugin and instructions arrays

Problem

The pruner (prune() in compaction.ts) replaces old tool outputs with "[Old tool result content cleared]" to save context window space. Currently, only the built-in skill tool is protected from this behavior via a hardcoded array.

Users who rely on MCP-based skill/context loaders (or any tool whose output must persist throughout the session) have no way to prevent their content from being silently erased after ~40k tokens of newer tool output accumulate. This causes the model to lose critical operational context mid-session.

Solution

Make the protection list configurable:

{
  "compaction": {
    "protectedTools": ["my_mcp_server_skill_loader", "another_context_tool"]
  }
}

Changes

File	Change
src/config/config.ts	Add protectedTools: z.array(z.string()).optional() to compaction schema
src/config/config.ts	Extend mergeConfigConcatArrays() to handle nested compaction.protectedTools array
src/session/compaction.ts	Read protectedTools from config and merge with built-in default ["skill"]

Design decisions

• Additive merge, not replace: User config extends the default list rather than replacing it, so "skill" is always protected even if not explicitly listed
• Follows existing pattern: plugin and instructions already use concat + dedup in mergeConfigConcatArrays() — this adds the same logic for a nested field
• Fully backward-compatible: Without explicit config, behavior is identical to current main
• Zero runtime cost when unused: Falls back to spread of empty array (?? [])