chore: purge committed build artifacts from packages (audit #13)

- Untrack dist/ bundles (axis-init, axis-server), python-sdk wheel/sdist,
  and egg-info; all proven to regenerate from source via each package's
  own build (8 of 11 byte-identical to the committed copies)
- Add prepublishOnly build hook to axis-init so publishing rebuilds its
  bin from source (axis-server already had one)
- Ignore rules for packages/*/dist, egg-info, wheels, venv landed in
  .gitignore alongside 56c373df

Files stay present on disk: the global axis-server binary resolves into
packages/axis-server/dist and live MCP sessions run from it.