fix(preflight): validate paths and input combinations before side-effecting steps

[Alphaxiaoteng]

Summary

1. Rejects invalid paths early before proxy startup or irreversible side effects (Fixes Invalid prompt-file and working-directory paths fail only after setup side effects #141): Added preflight validation to ensure non-empty prompt-file and working-directory paths exist and are accessible before installing packages, starting the Responses proxy, or running the irreversible drop-sudo step.
2. Fails early on mutually exclusive or invalid input combinations (Fixes Mutually exclusive inputs are rejected only after irreversible setup steps #139): Added preflight checks to immediately reject invalid combinations (e.g. prompt + prompt-file, output-schema + output-schema-file, permission-profile + sandbox, permission-profile with read-only) right after Node setup, preventing unexpected mutations to the runner before reporting input errors.

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Changes

• src/main.ts:
  • Added validate-inputs subcommand checking mutual exclusivity and path existence.
• action.yml:
  • Added Preflight validate inputs step immediately after actions/setup-node, executing before check-write-access, proxy installation, or drop-sudo.
• test/validateInputsPreflight.test.mjs:
  • Added 5 unit tests covering mutually exclusive input rejections, missing path failures, and successful preflights.

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Verification

• npm run build cleanly compiled with esbuild.
• npm test passed: 149/149 tests passing.

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[Alphaxiaoteng]

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