feat(console): manage source registries and discovery

[gbaudrit]

Summary

Delivers the Console experience for managing Source registry registrations, reviewing trust and freshness, discovering merged Source observations, and importing an exact version through the canonical Registry APIs. Closes #249.

Changes

• Add Platform Administrator Console pages for official, external, and private registry registrations, including CRUD, enable/disable, manual refresh, ETag conflict recovery, freshness, scheduling, errors, and last-known-good observations.
• Select authorized instance-scoped Secret references for private registry credentials without exposing Secret values.
• Add bounded registry discovery filters and grouped portable Source identities with shard-local latest markers, exact versions, digests, provenance, trust, verification, revocation, and conflict states.
• Add an exact-import confirmation flow that pins the registry registration, observation, catalog, opaque version, and manifest digest before delegating to the canonical import API.
• Add responsive Registry navigation and links to the existing Sources administration surface without duplicating Source or Channel refresh-policy UI.
• Add complete English and French resource catalogs, rendering and interaction coverage, a route-regression test, and Registry Console documentation.

Validation

• ./scripts/ci/verify-dotnet-format.ps1 -BaseRevision origin/main succeeded.
• dotnet build Agentstration.slnx --configuration Release --no-restore succeeded with 0 warnings and 0 errors.
• dotnet test Agentstration.slnx --configuration Release --no-build --verbosity minimal succeeded: 792 total, 788 passed, 4 optional integrations skipped, 0 failed.
• Desktop smoke test succeeded at 1440x900 against the local Development executable for the registry list, registration editor, and discovery pages, with no browser console errors.
• Mobile smoke test succeeded at 390x844 for the same routes; forms collapse to one column and wide tabs/tables remain horizontally scrollable, with no browser console errors.
• Registry Console, current capabilities, and Source concept documentation updated.

Breaking changes

None.