Rewrite AGENTS.md as an architecture guide and add CLAUDE.md

[jasonleenaylor]

Rewrites AGENTS.md as an architecture guide and adds CLAUDE.md.

Stacked on #400, which is stacked on #364. Review those first; this PR targets #400's branch.

Why rewrite rather than amend

The old file was an onboarding log rather than guidance. It inventoried the repository root
and the top-level directories, which the repository already states, and it transcribed the
SDK version, the target frameworks and the CI command sequence, which then drifted from
global.json, the .csproj files and ci-cd.yml. The architecture — the generation
pipeline, the model schema, the persistence design — was not described at all.

What the file now carries that cannot be discovered cheaply

• Editing a Generated*.cs file does not fail. Regeneration is skipped while the file is
  newer than MasterLCModel.xml, so the edit compiles into the local build and the tests
  pass, while CI generates from the XML into a fresh tree and builds different code. The files
  are gitignored, so the edit can never be committed. A fresh clone or worktree has none of
  them until the first build, and a stale one left by a branch switch is compiled as it stands.
• Per-rule consequences instead of one blanket claim. Rules 2, 3 and 5 corrupt data;
  rule 4 breaks the build; rule 1 does neither, which is what makes it dangerous.
• The FLEx Bridge metadata cache obligation from WARNING 4.
• Removing a worktree needs the directory left first, since build output is untracked and
  plain git worktree remove refuses.
• -p:ParallelizeAssembly=false is not optional: ICU and the writing system subsystems
  hold shared state.
• Windows builds need the C++ tools whether or not the IDE is used, because code
  generation preprocesses the IDL with cl.exe, located via vswhere.
• A root build compiles every project, so a stray .cs file from another branch produces
  errors that look like your change broke something.

Corrections

The IOC section described StructureMap. Since #393 the container is
Microsoft.Extensions.DependencyInjection, wrapped in MicrosoftServiceLocator so
GetInstance<T>() still works.

Facts now pointed at rather than copied

The SDK floor, target frameworks and CI sequence point at global.json, the .csproj files
and .github/workflows/ci-cd.yml. The transcriptions are what went stale; the definitions
cannot.

The -m:1 rule from #364 and the instruction to report failing instructions are carried
forward unchanged. The existing GitVersion guidance is also carried forward, with the two
checkouts that break it named alongside it.

Checked against source

The flid arithmetic (LexSenseTags.kflidDefinition is 5016005), LexDb atomic under
LangProject, Styles as an StStyle collection, all seven Overrides*.cs names, the nine
generated files, CheckWinForms in three projects, and every path in the layout tree.

CLAUDE.md imports AGENTS.md and points Claude-only procedures at .claude/skills/,
matching FieldWorks and interlinearizer-extension.

Docs only, so no build or test run is claimed.

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