Fix Step 7 handoff 400 by moving skills to the Activities leaf

[Pierre Malarme (pmalarme)]

Problem

A deployed Step 7 agent hit a runtime error after a handoff sequence (a specialist hands back to the Coordinator):

Error code: 400 - No tool call found for function call output with call_id ...

Root cause

The skills provider is a context provider that registers its skill tools on whichever agent holds it. Step 7 uses a HandoffBuilder runtime handoff, where the Coordinator is the only participant invoked twice — once to route, then again to synthesize after a specialist hands back. Every agent sets the hard workshop invariant default_options={"store": False}, so the framework replays the whole tool-call history on the Coordinator's second call. A prior change (PR #9) had moved the skills onto the Coordinator; that extra tool stream desyncs the replay and the service rejects it with the 400. Leaf specialists are invoked once, so they carry context providers safely.

Fix

• Step 7: Coordinator becomes a pure router (no tools, no context providers); the travel-guide (PDF) + response-guardrails skills ride on the Activities leaf. The Coordinator gathers flights/hotels first, then hands the full draft to Activities last so the deliverable and guardrail check reliably run through it.
• Steps 8–9: unchanged runtime — the workflow's finalize_itinerary node still owns the skills (invoked once, no handoff re-entry). Stripped the skills off the now-legacy build_travel_coordinator factory (reference-only, not the 8–9 runtime path) and corrected its docstring + the finalize prompt that referenced the Step 7 Coordinator owning the deliverable.
• Docs / README / slices / scaffold: kept in sync (docs > solutions > step_files). Adds the teaching point that a handoff Coordinator can't own tools/skills under store=False replay — which motivates Step 8's dedicated finalize node.

Keeps all the independent PR #9 improvements (Flights returns fares, Hotels web toolbox, multi-section prompts, model-size hint, agent-framework-orchestrations in requirements).

Known teaching limitation (intentional)

In Step 7 the guarded final path is routed, not structurally enforced — that's a real limitation of a pure router, and it's exactly what Step 8's workflow finalize node fixes.

Validation

• Custom parity check: all coordinator.py/workflow.py parse; every specialist slice == its *_INSTRUCTIONS constant; step_files/07 slices == solution.
• python .workshop/scripts/lint_steps.py → 0 failures (2 pre-existing unrelated warnings).
• Reviewed by the rubber-duck agent; its actionable findings (stale finalize-prompt reference, determinism of the deliverable path, over-broad root-cause wording, legacy-factory docstring) are addressed.