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. It recorded that dotnet build and dotnet test had
FAILED and attributed both to GitVersion, which is a dead end for anyone who reads it — the
real causes are a detached-HEAD worktree and a race in the code generation targets. It also
inventoried the repository root and top-level directories, which the repository already
states, and transcribed the SDK version and CI command sequence, which drifted from
global.json and ci-cd.yml.

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.
• Worktrees need a named branch — GitVersion cannot version a detached HEAD — and removal
  needs the directory left first, since build output is untracked.
• -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.

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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