feat(exec): support interpreter on serial and parallel steps

[jahvon]

Part 4/5. Stacked on #441.

Summary

Inline cmd steps in serial/parallel executables now take their own interpreter, so one workflow can mix shell and Python without splitting into separate executables.

serial:
  execs:
    - cmd: ./fetch-data.sh
    - cmd: |
        import json
        print(json.load(open("data.json"))["total"])
      interpreter: python

Notable Changes

• Both step configs $ref the same ExecInterpreter definition the exec type uses, so there is one enum rather than three parallel ones. (Part 1 hoisted it to a top-level definition for exactly this.)
• A step that omits interpreter runs under the shell as before; a ref step ignores the field — the referenced executable brings its own.
• This lifts the interim restriction from feat(mcp): add run_python tool #440: --interpreter now applies to every --cmd in an invocation, in serial and parallel mode alike.
• ExecutableForCmd takes the interpreter as a new parameter (5 call sites). Its unused int parameter is left alone to keep the diff to the one concern.

Testing

• E2E: a serial executable mixing a shell step and a Python step, each running under its own interpreter; a step without an interpreter staying on the shell (asserted via echo, a shell builtin, so it only passes if the step really stayed there); and --interpreter applied across a multi-command batch.
• The e2e case that previously asserted the multi-command rejection is replaced with one asserting it now works.
• flow validate passes; generate produces no diff.

🤖 Generated with Claude Code

https://claude.ai/code/session_01R328pa3FUUfga4gYah1iQi

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/internal/exec.go	83.33%	1 Missing ⚠️

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