Add pipeline resume with checkpoint persistence

[viamus]

Summary

• Automatically save a JSON checkpoint after each successful pipeline step, enabling resume on failure without re-running completed work
• Add --resume flag to pick up from the last failed step, and --from-step <name> to re-execute from a specific step
• Checkpoints stored in .codegenesis/{pipeline}.checkpoint.json next to the YAML file, deleted on successful completion
• Detect YAML changes via SHA-256 hash and warn when resuming a modified pipeline

Details

New files:

• Pipeline/PipelineCheckpoint.cs — Data model (version, yaml hash, completed steps, step outputs, metrics)
• Pipeline/CheckpointManager.cs — Atomic load/save/delete with SHA-256 hash computation

Modified files:

• Cli/RunPipelineCommandSettings.cs — --resume and --from-step flags with mutual exclusivity validation
• Pipeline/PipelineExecutor.cs — Explicit IStepExecutor interface impl (no resume) vs public overload (with resume params), skip logic, checkpoint save after each step
• Cli/RunPipelineCommand.cs — Checkpoint wiring, cached output restoration, resume hint on failure
• UI/PipelineRenderer.cs — RenderStepSkipped() (cached indicator) and RenderResumeHint() (tip message)
• Program.cs — DI registration + CLI help examples

Test plan

• Run a multi-step pipeline — verify .codegenesis/ checkpoint created after each step
• Force a step failure — verify checkpoint persists with FailedStepName
• Run with --resume — verify completed steps show "(cached)" and execution resumes from failed step
• Run with --from-step <name> — verify re-execution starts at the specified step
• Modify YAML between runs — verify warning about changed config
• Successful pipeline completion — verify checkpoint file is deleted
• Run --resume without checkpoint — verify clear error message
• Run --resume + --from-step together — verify validation error

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