Replace hardcoded string constants for Runtime and ServiceName types
with struct types loaded from an embedded registry.json file at init
time. This makes it possible to update supported versions without
code changes.

- Add registry.go and registry.json (embedded JSON data for all
  runtimes and services with their supported versions)
- Rewrite Runtime from string const to struct with Name, Type,
  Versions, Docs fields
- Rewrite ServiceName from string const to struct with Name, Type,
  Versions, Disk, Docs fields
- Update RuntimeForStack to look up runtimes by type string
- Update all callers to use runtime.Type string comparisons instead
  of const equality
- Remove version.go and generate_versions.go (superseded by registry)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- type.go: add nil check for Runtime in defer to prevent panic when
  RuntimeByTitle returns an error before Runtime is assigned
- registry.go: sort Runtimes and ServiceNames after loading from the
  map to ensure deterministic ordering
- runtime.go: move Locations field inside Web struct to match the JSON
  nesting (docs.web.locations)
- service_name.go: add json tags for relationship_name and
  min_disk_size so they unmarshal correctly from snake_case JSON keys

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Abstract the filesystem using Go's standard fs.FS interface and change
Platformify() to return a map[string][]byte of generated files instead
of writing them to disk. This enables callers (like source-integration-
apps) to operate on in-memory filesystems and handle file output
themselves.

Changes:
- internal/utils: all file functions take fs.FS as first parameter,
  use fs.WalkDir/fs.ReadFile instead of os equivalents
- internal/question/models: WorkingDirectory becomes fs.FS, add Cwd
  string field for the OS path
- internal/question/*: update all callers for new utils signatures
- platformifier: interface returns (map[string][]byte, error),
  generic/django/laravel collect files in map instead of writing
- platformifier: remove custom FS interface (fs.go), mock (fs_mock_test),
  mock generator (platformifier_mock_test), and nextjs platformifier
- platformifier: adapt all tests to use fstest.MapFS and new interface
- commands/platformify.go: write returned file map to disk
- validator: use os.DirFS for FindAllFiles call

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- utils.go: use "/" instead of os.PathSeparator in FindFile sort,
  since fs.WalkDir always returns forward-slash paths
- stack.go: fix keywords type assertion from []string to []any,
  since JSON arrays unmarshal as []any
- laravel.go: return empty map instead of nil for consistency
  with other platformifiers

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>