Bump AWF to v0.28.4 and remove compiler `auto` pricing overlay

[lpcox]

Context

PR #54364 adds a scoped, conservative github-copilot/auto pricing overlay to compiled AWF configuration so Copilot's valid server-side auto selector can pass the current AWF AI-credits pre-request guard.

This is an appropriate compatibility workaround, but model accounting policy ultimately belongs in the API proxy. Native support is tracked in github/gh-aw-firewall#7603.

Follow-up

Once an AWF release provides first-class accounting for recognized dynamic selectors:

• bump gh-aw's pinned AWF/firewall version and image digests to v0.28.4, updating the minimum supported version as appropriate;
• remove the compiler-generated github-copilot/auto provider pricing overlay and its feature/version gating;
• remove the associated conservative catalog-maximum pricing calculation and generated lock-file entries;
• retain compatibility behavior only for explicitly supported older AWF pins, if required;
• add an integration/contract test proving that zero-config Copilot auto works with maxAiCredits using native AWF behavior;
• verify concrete response models use their own pricing and omitted/dynamic response metadata cannot undercount;
• document the remaining behavior of maxModelMultiplierCap for dynamic selectors.

Dependency

Blocked by github/gh-aw-firewall#7603 and adoption of AWF/firewall v0.28.4.