fix derive CLI version from package metadata

[NianJiuZst]

Summary

• add a shared CLI_VERSION sourced from package.json when no build override is present
• use the shared version for --version, User-Agent, and the update command
• make build:dev reuse build.ts so it no longer hardcodes an old version
• add regression coverage for dev-mode version output and User-Agent

Root cause

Several source paths fell back to stale hardcoded versions, including 1.0.4, while package.json is currently 1.0.12. The development build script also injected the stale version explicitly.

Validation

• bun run dev --version
• bun test test/version.test.ts test/client/http.test.ts
• bun run typecheck
• bunx eslint src/version.ts src/main.ts src/client/http.ts src/commands/update.ts build.ts test/version.test.ts test/client/http.test.ts
• bun run build:dev
• node dist/mmx.mjs --version
• bun test

Note: full repository lint still has pre-existing failures in unrelated tests: test/auth/timeout-fix.test.ts and test/client/stream.test.ts.