Add --dry-run mode to renderflow build

[Copilot]

Pipeline execution had no way to preview behavior without side effects — no safe way to validate config or debug output paths before committing a full build.

Changes

• src/cli.rs — Adds --dry-run flag to the Build subcommand
• src/main.rs — Threads dry_run into commands::build::run()
• src/commands/build.rs — Core dry-run logic:
  • Skips ensure_output_dir() — no directories created
  • Runs transforms (pure in-memory, no side effects) for accurate preview
  • Skips select_strategy() / pipeline.run_steps() — pandoc never invoked
  • Logs [dry-run] Would render <format> output to: <path> per format
  • Finishes with ✔ Dry-run complete — no output written
• renderflow/cli.py — Adds --dry-run Typer option with appropriate output
• tests/test_cli.py / src/commands/build.rs tests — Coverage for success path, no-file-creation guarantee, and config error propagation in dry-run mode

Usage

renderflow build --dry-run
renderflow build --dry-run --config custom.yaml

Original prompt

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This section details on the original issue you should resolve

<issue_title>⚡ Add dry-run mode for pipeline execution</issue_title>
<issue_description>Title: ⚡ Add dry-run mode for pipeline execution

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Description

Add a mode that simulates execution without actually running commands.

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Goals

• Allow users to preview pipeline behavior
• Improve safety and debugging

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Requirements

• Add flag:

renderflow build --dry-run

• Behavior:
  • log what would happen
  • do NOT execute commands
  • do NOT create files

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Acceptance Criteria

• Dry-run mode logs intended actions
• No files created
• No external commands executed</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ⚡ Add dry-run mode for pipeline execution #32

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Pull Request Summary by devActivity

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• @szmyty
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