Move planned work from docs to prioritized GitHub issues

[Brandon Werner (brandwe)]

Summary

• creates a prioritized backlog across Executive Demo, Prototype Hardening, and Federation Expansion milestones
• removes the superseded GitHub and Google next-* roadmap documents
• replaces actionable TODO and future-work text with links to GitHub issues
• reconciles ADR-010 status and clarifies the current Conditional Access implementation boundary
• documents that planned work belongs in GitHub Issues

Backlog

• Executive demo epic: Epic: Rehydrate the Azure executive demo #38
• P0 execution issues: Fix service-local sanitize_token_error import regression #24, Create a clean scoped Azure deployment environment #25, Deploy and validate the Azure-only enforcement stack #26, Repair deployment status and stale-state detection #27
• Prototype hardening: Persist SPIRE server CA and registration state #28, Replace single-use join-token attestation for Container Apps #29, Replace VM run-command orchestration with SPIRE APIs #30, Complete and accurately document Conditional Access enforcement #31, Harden SPIRE networking and deployment secret storage #32, Clear dependency and GitHub Actions hardening backlog #33
• Federation expansion: Validate and productize GitHub Actions federation #34, Validate and productize Google Cloud federation #35, Generalize dynamic agent onboarding #36

Validation

• git diff --check
• Python compile checks for edited CA evaluator modules
• go test ./internal/ca
• mkdocs.yml parses and every navigation target exists
• deleted roadmap references are absent

A full MkDocs build could not be run locally because PyPI timed out while fetching MkDocs; the Docs workflow will run on this PR.

Closes #37