Update hooks.md and session.md for consolidated HookContext

- HookEventContext renamed to HookContext (used for events and commands)
- RegisteredCommand.handler: (ctx) -> (args, ctx)
- before_compact: previousCompactions -> branchEntries, model moved to ctx.model
- ctx.exec -> pi.exec in examples
- ctx.sessionFile -> ctx.sessionManager.getSessionFile()
- CompactionPreparation: document turnPrefixMessages, isSplitTurn, previousSummary
- session.md: clarify details field for compaction/branch summary

packages/coding-agent/docs/session.md

@@ -89,19 +89,19 @@ Created when context is compacted. Stores a summary of earlier messages.
 ```
 
 Optional fields:
-- `details`: Hook-specific data (e.g., ArtifactIndex for structured compaction)
+- `details`: Compaction-implementation specific data (e.g., file operations for default implementation, or custom data for custom hook implementations)
 - `fromHook`: `true` if generated by a hook, `false`/`undefined` if pi-generated
 
 ### BranchSummaryEntry
 
-Created when switching branches via `/tree`. Captures context from the abandoned path.
+Created when switching branches via `/tree` with an LLM generated summary of the left branch up to the common ancestor. Captures context from the abandoned path.
 
 ```json
 {"type":"branch_summary","id":"g7h8i9j0","parentId":"a1b2c3d4","timestamp":"2024-12-03T14:15:00.000Z","fromId":"f6g7h8i9","summary":"Branch explored approach A..."}
 ```
 
 Optional fields:
-- `details`: File tracking data (`{ readFiles: string[], modifiedFiles: string[] }`)
+- `details`: File tracking data (`{ readFiles: string[], modifiedFiles: string[] }`) for default implementation, arbitrary for custom implementation
 - `fromHook`: `true` if generated by a hook
 
 ### CustomEntry
@@ -172,7 +172,7 @@ const lines = readFileSync("session.jsonl", "utf8").trim().split("\n");
 
 for (const line of lines) {
   const entry = JSON.parse(line);
-  
+
   switch (entry.type) {
     case "session":
       console.log(`Session v${entry.version ?? 1}: ${entry.id}`);