Use CompactionResult type for hook compaction return value

- Import CompactionResult in hooks/types.ts
- Replace inline type with CompactionResult for SessionEventResult.compaction
- Add labels feature to changelog

packages/coding-agent/CHANGELOG.md

@@ -30,7 +30,8 @@
 
 - **Entry IDs**: Session entries now use short 8-character hex IDs instead of full UUIDs
 - **API key priority**: `ANTHROPIC_OAUTH_TOKEN` now takes precedence over `ANTHROPIC_API_KEY`
-- **New entry types**: `BranchSummaryEntry` for branch context, `CustomEntry` for hook data
+- **New entry types**: `BranchSummaryEntry` for branch context, `CustomEntry` for hook data, `LabelEntry` for user-defined bookmarks
+- **Entry labels**: New `getLabel(id)` and `appendLabelChange(targetId, label)` methods for labeling entries. Labels are included in `SessionTreeNode` for UI/export.
 
 ### Fixed
 

packages/coding-agent/docs/session-tree-plan.md

@@ -91,6 +91,30 @@ Questions to resolve:
 - [ ] Display labels in UI (tree view, path view) - deferred to UI phase
 - [ ] `/label` command - deferred to UI phase
 
+### CustomMessageEntry<T>
+
+Hooks can define their own custom message entry types and inject them into the session.
+
+```typescript
+export interface CustomMessageEntry<T = unknown> extends SessionEntryBase {
+  type: "custom_message";
+  customType: string;           // Hook identifier
+  content: (string | Attachment)[];  // Message content
+  details?: T;                  // Hook-specific data (like tool result details)
+  display: boolean;             // Whether to display in TUI
+}
+```
+
+Behavior:
+- [ ] Participates in context and compaction as user messages (after messageTransformer)
+- [ ] Not displayed as user messages in TUI
+- [ ] Display options:
+  - `display: false` - hidden entirely
+  - `display: true` - baseline renderer (content with different bg/fg color)
+  - Custom renderer defined by the hook that contributes it
+- [ ] Define injection mechanism for hooks to add CustomMessageEntry
+- [ ] Hook registration for custom renderers
+
 ### HTML Export
 
 - [ ] Add collapsible sidebar showing full tree structure

packages/coding-agent/src/core/hooks/types.ts

@@ -7,7 +7,7 @@
 
 import type { AppMessage, Attachment } from "@mariozechner/pi-agent-core";
 import type { ImageContent, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
-import type { CutPointResult } from "../compaction.js";
+import type { CompactionResult, CutPointResult } from "../compaction.js";
 import type { CompactionEntry, SessionEntry } from "../session-manager.js";
 import type {
 	BashToolDetails,
@@ -354,11 +354,7 @@ export interface SessionEventResult {
 	/** If true (for before_branch only), skip restoring conversation to branch point while still creating the branched session file */
 	skipConversationRestore?: boolean;
 	/** Custom compaction result (for before_compact event) - SessionManager adds id/parentId */
-	compaction?: {
+	compaction?: CompactionResult;
-		summary: string;
-		firstKeptEntryId: string;
-		tokensBefore: number;
-	};
 }
 
 // ============================================================================