feat: add CLI-server protocol version mismatch detection

- Define PROTOCOL_VERSION constant in shared/src/version.ts
- Server sets X-Hapi-Protocol-Version header on /cli/* responses
- Server includes protocolVersion in /health endpoint
- CLI extracts serverProtocolVersion from API responses
- CLI shows version mismatch hints in both directions
- Add tests for error utilities and version extraction

close #104

shared/src/index.ts

@@ -3,4 +3,5 @@ export * from './modes'
 export * from './socket'
 export * from './sessionSummary'
 export * from './utils'
+export * from './version'
 export type * from './types'

shared/src/version.ts

@@ -0,0 +1,2 @@
+/** Bump when CLI↔Server REST/Socket API changes in a breaking way. */
+export const PROTOCOL_VERSION = 1