Add CustomMessageEntry rendering infrastructure

- Add renderCustomMessage to HookAPI for registering custom renderers
- Add CustomMessageRenderer type and CustomMessageRenderOptions
- Store customMessageRenderers in LoadedHook
- Add getCustomMessageRenderer(customType) to HookRunner
- SessionContext.entries now aligned with messages (same length, corresponding indices)

TUI can now correlate messages with their source entries to identify
custom_message entries and use hook-provided renderers.

packages/coding-agent/test/compaction-hooks.test.ts

@@ -63,6 +63,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			path: "test-hook",
 			resolvedPath: "/test/test-hook.ts",
 			handlers,
+			customMessageRenderers: new Map(),
 			setSendHandler: () => {},
 		};
 	}
@@ -238,6 +239,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
+			customMessageRenderers: new Map(),
 			setSendHandler: () => {},
 		};
 
@@ -281,6 +283,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
+			customMessageRenderers: new Map(),
 			setSendHandler: () => {},
 		};
 
@@ -303,6 +306,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
+			customMessageRenderers: new Map(),
 			setSendHandler: () => {},
 		};