Migrate extensions to FScript and remove legacy extension projects

[pchalamet]

Summary

• migrate built-in extensions to bundled FScript scripts under src/Terrabuild/Scripts
• integrate descriptor-based FScript invocation in Terrabuild.Scripting
• move extension protocol/contracts into Terrabuild.Scripting
• remove Terrabuild.Extensions and Terrabuild.Extensibility
• update core wiring (Configuration, GraphPipeline/Node, GraphPipeline/Batch) to new scripting contract
• add/expand scripting tests for exports, flags, dispatch/default resolution, and required arg behavior
• update DocGen integration for script-based docs parity
• support workspace-local and HTTPS script sources

Structural changes

• added script registry: src/Terrabuild/Scripts/Registry.fs
• added protocol/template doc: src/Terrabuild/Scripts/EXTENSION-PROTOCOL.md
• removed projects:
  • src/Terrabuild.Extensions/**
  • src/Terrabuild.Extensibility/**

Validation

• make self passes
• make smoke-tests passes
• scripting tests pass

Notes

• built-in extensions keep @... identifiers
• extension scripts now follow context-first protocol with descriptor flags (dispatch, default, batchable, cacheability)