Consolidate agent guidance, remove legacy compatibility APIs, add generator validate mode, and update templates/tests

[0x5143]

Motivation

• Consolidate and stabilize agent-facing guidance by making AGENTS.md the canonical agent engineering guide and avoiding duplicated/marketing content in CLAUDE.md and other docs.
• Remove long-deprecated callback/compatibility surface and ambiguous API aliases to reduce maintenance burden and avoid accidental usage.
• Add a validate-only generation mode so the generator can be used in CI to parse/validate/render templates without writing outputs.
• Update code-gen templates and tests to match the modern runtime API (LoadAsync/GetCached/IsLoaded) and the new runtime surface.

Description

• Rewrote AGENTS.md into a concise agent-focused engineering guide and shortened CLAUDE.md to point to it, and added planning/ADR indexes under docs/ (docs/adr/, docs/planning/).
• Added a validate CLI command and GenerationMode.ValidateOnly to DataTables.Generator, changed generator logic to render/validate without emitting files when in validate mode, and exposed ValidateOnly on the MSBuild task.
• Updated generator templates (*.tt / generated *.cs) and template renderers to stop using legacy GetDataTableInternal and instead use DataTableManager.GetCached, and removed generation of obsolete compatibility helper methods.
• Modernized runtime public surface by removing legacy callback/compatibility APIs and interfaces (obsolete CreateDataTable* callbacks, IDataTableManager, compatibility properties/methods), renaming cache stats fields to estimated-budget names, and requiring context-aware table registrations; adjusted DataTableContext/DataTableManager accordingly.
• Updated sandbox-generated code and numerous unit tests and test fixtures to reflect the new API shapes and added tests that cover validate-only generation and documentation smoke checks.

Testing

• Ran the unit test suite dotnet test tests/DataTables.Tests/DataTables.Tests.csproj (including PublicApiMetadataTests, DocumentationSmokeTests, GenerationTransactionTests, generator/schema tests and concurrency tests) and all tests passed.
• Exercised the generator CLI help and the new dotnet dtgen validate --help invocation to confirm long-option docs and validate-mode help are available and consistent.
• Ran the new validate-only generation integration tests (template/schema rendering without writing outputs) and they passed, confirming no files are emitted in ValidateOnly mode.
• Verified updated code-gen templates compile via the documentation and template compilation smoke tests.

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Codex Task