docs(site): document custom steps injection in front-matter reference

[github-actions]

Summary

• Added dedicated Custom Steps Injection section to front-matter.mdx
• Documents steps, post-steps, setup, and teardown fields with execution order, use cases, and practical examples
• Clarified env: field status (reserved, not yet used by compiler)

Changes

• site/src/content/docs/reference/front-matter.mdx:
  • Added new Custom Steps Injection section after Repositories
  • Mermaid execution flow diagram showing Setup → Agent → Executor → Teardown job sequence
  • Table summarizing each field's job placement and use cases
  • Dedicated subsection for each field (steps, post-steps, setup, teardown) with YAML examples
  • Job condition rules explaining when each field's steps execute
  • Full end-to-end example showing all four fields in a realistic data-pipeline workflow
  • Updated env: comment to clarify it's defined but not yet used by the compiler (directs users to engine.env or mcp-servers.<name>.env for process-specific vars)

Accuracy checks

• Verified execution order against src/compile/common.rs (generate_prepare_steps, generate_finalize_steps, generate_setup_job, generate_teardown_job)
• Confirmed all four fields are actively used:
  • front_matter.steps → generate_prepare_steps() (line 3129)
  • front_matter.post_steps → generate_finalize_steps() (line 3130)
  • front_matter.setup → generate_setup_job() (line 3113)
  • front_matter.teardown → generate_teardown_job() (line 3121)
• Verified env: field exists in FrontMatter struct (line 716) but is never referenced in compiler code
• Checked job condition behavior matches generated pipeline structure

Validation

• cd site && npm ci && npm run build
• All internal links valid (starlight-links-validator passed)
• 32 pages built successfully
• Mermaid diagram renders correctly

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