feat(coding-agent): add session naming via /name command and extension API

- Add SessionInfoEntry type for session metadata
- Add /name <name> command to set session display name
- Add pi.setSessionName() and pi.getSessionName() extension API
- Session selector shows name (in warning color) instead of first message when set
- Session name included in fuzzy search
- /session command displays name when set

closes #650

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

@@ -108,6 +108,8 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 				sendMessage: async () => {},
 				sendUserMessage: async () => {},
 				appendEntry: async () => {},
+				setSessionName: () => {},
+				getSessionName: () => undefined,
 				getActiveTools: () => [],
 				getAllTools: () => [],
 				setActiveTools: () => {},