Adds check 29: ported plugin repos must not carry a `-buildmode=plugin` build
target (the legacy .so compile the wasm migration retires). Lands DISABLED
behind a per-check const toggle (enableBuildmodePluginCheck=false) so it emits
no output and never fails while the fleet is still porting; WO-WZ-017 flips it
on. Scans plugin Makefiles/*.sh only. Unit-tested; golden snapshots unaffected.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
