Refactor Step 7: deliver specialist slices, de-code the doc, add coordinator hints

[Pierre Malarme (pmalarme)]

What & why

Refactors Step 7 (multi-agent handoff) so learners read the per-specialist config slices we deliver and translate them into the coordinator.py scaffold, rather than authoring slices themselves. The step doc no longer contains implementation code — it points at the slices and the TODOs.

Changes

• Learn more: added the Agent Framework handoff_workflow_as_agent.py sample bullet (Step 7 previously had no upstream-sample link).
• Delivered slices: ship complete agents/{flights,hotels,activities}/agent.yaml + agent.manifest.yaml (byte-identical to the solution) as the reviewable contract; learners read, don't author them.
• De-coded doc: removed inline YAML/Python; Section 1 is "Read the specialist config slices", Section 2 sends learners to fill the coordinator.py TODOs, Section 3 is a prose in-place main.py edit.
• Coordinator hints: added COORDINATOR_INSTRUCTIONS guidance (Role / Routing rules / Full-trip / Etiquette) in both the doc and the scaffold, since the Coordinator has no slice.
• Foundry skill framed as optional: the doc explains the checked-in solution is the Foundry-enabled reference; learners who skipped the Foundry skill in Step 6 carry their local-only skills provider forward.
• Scaffold: sharpened TODOs to point at specific slices; added load_dotenv(override=True) to match the solution as main.py collapses.
• Boy-Scout cleanup: removed 6 redundant .gitkeep files from now-populated dirs.

Validation

• python .workshop/scripts/lint_steps.py — 0 failures (2 pre-existing unrelated warnings)
• python -m pytest .workshop/scripts/tests — 78 passed