[Story]: Policy inheritance and override resolution

## Summary
Implement policy inheritance and override resolution so a policy (model access, budget, residency) set at a BU node applies to all descendant teams unless explicitly overridden lower in the tree.

## Intent / Source of truth
Buyers expect per-node policy with inheritance, not per-team manual configuration at 10k-seat scale. Part of [Epic] Organization hierarchy and delegated administration.

## Scope
- [ ] Policy resolution algorithm (nearest-ancestor-wins, or documented alternative)
- [ ] Support for model-access, budget, and residency policy types
- [ ] Explicit override recorded and distinguishable from inherited defaults
- [ ] Resolution result exposed via API for debugging ("why does this team have this policy")

## Out of scope
- Policy authoring UI

## Verification
Test matrix covering inherit / override / multi-level-override cases for each policy type; a "why" debug endpoint returns the correct resolution chain.

## Risk assessment
Silent policy resolution bugs (e.g. an override not taking effect) directly cause either budget leaks or access-denial incidents; needs strong test coverage before any customer relies on it.

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation and enterprise-readiness research. A human owns intent, verification and consequences.

No files, tests, or API entry points are named. Start by locating the existing organization-tree and policy representations, then document the nearest-ancestor resolution contract before implementing model-access, budget, and residency inheritance. Add the requested inherit, override, and multi-level test matrix, and verify that the “why” endpoint returns the resolution chain.

由索引模型根据 Issue 内容生成。