Add design-notes migration plan with document triage and proposed directory structure

[Copilot]

Description

Reviews all 62 design documents in the design-notes repository and produces a migration plan identifying which should be migrated to the main repo. Also proposes an improved topic-based directory structure.

Result: 36 documents recommended for migration, 27 excluded.

• Migration criteria: implemented in current codebase, still relevant, not related to Applications.Core
• Exclusions by category:
  • 8 aspirational/not yet implemented (ACI, authz, serverless, offline install, etc.)
  • 12 Applications.Core-related (Applications.Core/*, Applications.Dapr/*, Applications.Datastores/*, Applications.Messaging/*)
  • 4 operational/templates, 2 spec-kit artifacts, 1 empty directory
• Proposed structure: topic-based grouping under docs/design-notes/ with 9 categories: architecture/, cli/, extensibility/, gitops/, guides/, recipes/, security/, tools/, ucp/
• Key structural change: replaces the original repo's mixed-concern directories (e.g., resources/ contained both Applications.Core and recipe designs) with topic-aligned grouping
• Extensibility: User-defined types, resource extensibility, and compute extensibility (recipe-backed core resource types including containers, routes, recipe packs) are grouped under a single extensibility/ category (9 documents)
• Security: Includes threat models and the Radius.Security/secrets sensitive data redaction design under security/ (3 documents)
• GitOps: Includes the Flux Controller technical design alongside the feature spec and DeploymentTemplate controller under gitops/ (3 documents)

Each recommendation includes a summary and evidence of implementation (specific pkg/ paths).

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Yes
  • Not applicable

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[nicolejms]

recipe-backed core resources has been implemented. Those documents need to be migrated.

[Copilot]

recipe-backed core resources has been implemented. Those documents need to be migrated.

Moved 6 documents to the migration list in 2371ed9:

• architecture/2025-04-compute-extensibility.md → new compute/ category
• features/2025-06-compute-extensibility-feature-spec.md → compute/
• features/2025-08-29-container-resource-type.md → compute/
• features/2025-09-02-routes-resource-type.md → compute/
• recipe/2025-08-recipe-packs.md → recipes/
• recipe/2025-09-container.md → recipes/

Verified implementation evidence: Radius.Compute/containers and routes in deploy/manifest/built-in-providers/dev/radius_compute.yaml, RecipePack in typespec/Radius.Core/recipePacks.tsp and pkg/corerp/frontend/controller/recipepacks/. Updated totals to 34 recommended, 28 excluded.

[nicolejms]

@copilot merge both extensibility sections

[Copilot]

Merged both into a single "Extensibility (9 documents)" section in ef8ca39. The compute/ directory is removed from the proposed structure; all extensibility docs (UDTs + compute extensibility) are now under extensibility/.

[nicolejms]

@copilot this is implemented, migrate the doc

[Copilot]

Moved to the migration list under Security in ef8ca39. Evidence: Radius.Security/secrets resource type is defined in deploy/manifest/built-in-providers/dev/radius_security.yaml.

[nicolejms]

@copilot migrate this doc

[Copilot]

Moved to the migration list under GitOps in ef8ca39. Evidence: Flux controller in pkg/controller/reconciler/flux_controller.go, predicate in flux_gitrepository_predicate.go, and functional tests in test/functional-portable/kubernetes/noncloud/flux_test.go.