Strengthen source-control provider foundation

[ColeMurray]

Summary

• add deployment-level SCM_PROVIDER resolution with a default of github
• route provider selection through the source-control factory and return explicit 501 for unimplemented providers on non-public routes
• document single-provider boundaries with an ADR and a provider contribution checklist
• add focused tests for provider env resolution and provider factory behavior

Validation

• npm run typecheck -w @open-inspect/control-plane
• npm -w @open-inspect/control-plane exec vitest run src/source-control/config.test.ts src/source-control/providers/index.test.ts

[github-actions]

Terraform Validation Results

Step	Status
Format	⚠️
Init	✅
Validate	✅

Note: Terraform plan was skipped because secrets are not configured. This is expected for external contributors. See docs/GETTING_STARTED.md for setup instructions.

Pushed by: @ColeMurray, Action: pull_request

[greptile-apps]

Greptile Overview

Greptile Summary

This PR establishes a clean architectural foundation for source control provider abstraction with single-provider-per-deployment constraints. The implementation adds deployment-level SCM_PROVIDER configuration (defaulting to github), routes provider selection through a factory with explicit 501 Not Implemented responses for unimplemented providers on non-public routes, and documents the single-provider boundaries through an ADR and contribution checklist.

• Added resolveScmProviderFromEnv() function that normalizes provider names and validates against supported values (github, bitbucket)
• Modified router to enforce provider implementation check via enforceImplementedScmProvider() middleware
• Updated createSourceControlProvider() factory to throw explicit errors for bitbucket (not yet implemented) vs unknown providers
• Added comprehensive tests for provider resolution and factory behavior
• Created ADR 0001 documenting the single-provider deployment decision and boundary rules
• Added provider contribution checklist to guide future SCM integrations

[github-actions]

Terraform Validation Results

Step	Status
Format	⚠️
Init	✅
Validate	✅

Note: Terraform plan was skipped because secrets are not configured. This is expected for external contributors. See docs/GETTING_STARTED.md for setup instructions.

Pushed by: @ColeMurray, Action: pull_request