Refactor: shared exec utility, rename CustomMessageRenderer to HookMessageRenderer

- Extract execCommand to src/core/exec.ts, shared by hooks and custom-tools
- Rename CustomMessageRenderer -> HookMessageRenderer
- Rename registerCustomMessageRenderer -> registerMessageRenderer
- Renderer now receives HookMessage instead of CustomMessageEntry
- CustomMessageComponent now has setExpanded() and responds to Ctrl+E toggle
- Re-export ExecOptions/ExecResult from exec.ts for backward compatibility

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

@@ -63,7 +63,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			path: "test-hook",
 			resolvedPath: "/test/test-hook.ts",
 			handlers,
-			customMessageRenderers: new Map(),
+			messageRenderers: new Map(),
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
@@ -240,7 +240,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
-			customMessageRenderers: new Map(),
+			messageRenderers: new Map(),
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
@@ -286,7 +286,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
-			customMessageRenderers: new Map(),
+			messageRenderers: new Map(),
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
@@ -311,7 +311,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 					],
 				],
 			]),
-			customMessageRenderers: new Map(),
+			messageRenderers: new Map(),
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},