Implement PipelineActionStore

[philbrookes]

Description

Implement a new store for pipeline actions in internal/extension/registry.go. Actions are ordered per-policy, keyed by {PolicyResourceID, Phase, Index}. Index allocation is atomic per (Policy, Phase) pair using a mutex-protected counter. Cleaned up via ClearPolicyData.

Acceptance Criteria

• PipelineActionStore implemented with append and query operations
• Atomic index allocation per (Policy, Phase) — concurrent calls cannot produce duplicate indices
• ClearPolicyData clears all pipeline actions for a policy
• Unit tests

References

• Design: extensions-SDK-action-pipeline-design.md
• Parent: Extension SDK Action Pipeline #1898