`ogma-core`: `mergeSpecs` duplicates second spec's external variables instead of merging both

[philphauler]

Hey 👋

Was reading through the source and noticed a copy-paste slip in ogma-core/src/Command/Common.hs, line 101:

mergeSpecs s1 s2 = Spec
  { internalVariables = internalVariables s1 ++ internalVariables s2
  , externalVariables = externalVariables s2 ++ externalVariables s2  -- <- s2 twice
  , requirements      = requirements s1 ++ requirements s2
  }

externalVariables uses s2 ++ s2 instead of s1 ++ s2, so when combining multiple --input-file arguments via combineInputFiles, external variables from the first spec get silently dropped.

This affects all four backends (cFS, F Prime, ROS, standalone) since they all go through combineInputFiles.

One-character fix: s2 → s1.

Also spotted a small typo nearby in Standalone.hs:256 — "canbot" should be "cannot".

Happy to open a PR with both fixes 🙏