Adds X-Agnes-Latest-Version and X-Agnes-Min-Version headers to every
/api/* response. CLI consumes these to hard-stop on incompatible drift.
MIN_COMPAT_CLI_VERSION ships at 0.0.0 — no enforcement until a deliberate
wire-protocol break bumps it.
Also dedupes app version logic: app/main.py:_app_version() helper deleted,
replaced by app/version.py:APP_VERSION as the single source of truth.
test_app_version.py rewritten to target app.version.
