feat: Add dynamic workflow support

[tadhgdowdall]

Summary

This PR is linked to #47

Introduces deferred target evaluation for Python and TypeScript workflows. Dynamic targets are represented as placeholders during pipeline rendering and expanded into concrete steps when local execution reaches them.

What’s Done

• Added static and dynamic evaluation variants to pipeline IR.

• Added Python @hm.target(dynamic=True) support.

• Added TypeScript target(..., { dynamic: true }) support.

• Added runtime target evaluation for both DSL engines.

• Added hm.env() and env() helpers for runtime conditionals.

• Added local execution of dynamically generated steps.

• Supports:

  • Single generated steps
  • Linear command chains
  • Groups with multiple branches
  • Explicit group continuation
  • Snapshot continuation from one terminal step
  • Fresh-image continuation for multiple terminal steps

• Preserved caching, environment, image, and runner metadata.

• Added IR, SDK, engine, and executor tests.

Example

@hm.target(dynamic=True)
def choose_build() -> hm.Step:
    if hm.env("BUILD_TYPE") == "release":
        return hm.sh("npm run build:release")

    return hm.sh("npm run build:debug")

const chooseBuild = target(
  "choose-build",
  () =>
    env("BUILD_TYPE") === "release"
      ? sh("npm run build:release")
      : sh("npm run build:debug"),
  { dynamic: true },
);

Remaining Work

• Execute independent branches inside dynamic groups concurrently.
• Add committed CLI and Docker end-to-end tests.
• Improve build event and step-count reporting for generated steps.
• Define and implement cloud-side dynamic evaluation.
• Document dynamic targets and group continuation semantics.
• Confirm the proposed API and multiple-terminal behavior with maintainers.

Current Limitations

• Dynamic execution currently works only with the local backend.
• Steps inside dynamic groups execute sequentially.
• Initial build step counts include the placeholder rather than the eventual generated steps.
• A dynamic group with multiple terminal leaves cannot provide one snapshot for a subsequent step. The subsequent step therefore starts from the pipeline’s default image unless the group defines an explicit merged continuation.