fix: providers --force doesn't delete custom profiles (parity gap)

[robbycochran]

Problem

harness providers --force in Go only deletes the three named providers (github, vertex-local, atlassian) but does NOT delete custom profiles before reimporting. The bash providers.sh explicitly iterates sandbox/profiles/*.yaml, extracts each profile ID, and runs provider profile delete before reimporting.

Impact: --force in Go silently fails to update custom profiles that already exist — the import skips them.

Fix

1. Add ProviderProfileDelete(id string) error to the Gateway interface
2. In cmd/providers.go force path: iterate sandbox/profiles/*.yaml, parse id: field, call ProviderProfileDelete(id) before reimporting
3. Match bash behavior at providers.sh lines 57-63

Severity: High

Found by: 5-dimension code audit (equivalence review)