[Phase 3.6] Create advisor-documenter agent

[fernandoxavier02]

Task

• Create agents/advisor-documenter.md (Haiku)
• After each phase: updates the pipeline spec document in .specs/pipelines/
• Records: what ran, output produced, gate decisions, timing
• On pipeline completion: generates execution summary table

Acceptance Criteria

• Agent at agents/advisor-documenter.md
• Updates pipeline spec document after each phase
• Records: result, gate decision, duration, tokens
• Final summary table with all phases

Design Ref

Phase 3, task 3.6