feat(router): add image policy presets

[typelicious]

What changed

• add optional image.policy_tags to image-provider metadata
• allow image requests to carry image-policy hints via metadata, form fields, or X-FoundryGate-Image-Policy
• feed image-policy hints into modality-aware ranking so matching providers are preferred
• expose image-policy details in image dry-run diagnostics and docs
• rebase cleanly on top of the merged image workflow refinements from PR feat(api): refine image request validation #37

Why

Image routing now understands size and output fit, but operators still need a lightweight way to express intent such as quality, cost, batch, or editing without building a second policy layer.

How verified

• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_route_introspection.py tests/test_capabilities.py
• ./.venv-check-313/bin/ruff check foundrygate/main.py foundrygate/router.py foundrygate/config.py tests/test_route_introspection.py tests/test_capabilities.py
• /usr/bin/git diff --check