Make CompactionEntry and CompactionResult generic with details field

- CompactionEntry<T> and CompactionResult<T> now have optional details?: T
- appendCompaction() accepts optional details parameter
- Hooks can return compaction.details to store custom data
- Enables structured compaction with ArtifactIndex (see #314)
- Fix CompactionResult export location (now from compaction.ts)
- Update plan with remaining compaction refactor items

packages/coding-agent/src/core/index.ts

@@ -7,12 +7,12 @@ export {
 	type AgentSessionConfig,
 	type AgentSessionEvent,
 	type AgentSessionEventListener,
-	type CompactionResult,
 	type ModelCycleResult,
 	type PromptOptions,
 	type SessionStats,
 } from "./agent-session.js";
 export { type BashExecutorOptions, type BashResult, executeBash } from "./bash-executor.js";
+export type { CompactionResult } from "./compaction.js";
 export {
 	type CustomAgentTool,
 	type CustomToolFactory,