Architecture brief for AI agents — current and future state of Fallout

## Why
AI tools work better with a clear, machine-readable view of the codebase's module boundaries, layering, and intended evolution. Today this is spread across `docs/architecture.md`, `docs/rebrand-plan.md`, and the roadmap — not in a form an agent can orient on without reading the whole docs tree.

A purpose-built brief would let any AI tool:
- Understand current module boundaries (`Fallout.Common`, `Fallout.Build`, `Fallout.Components`, `Fallout.Tooling`, …) and how they layer.
- Know which abstractions are stable vs in-flight (the `internal` middleware pipeline in v11 vs the public `Fallout.Plugin.Sdk` in v12).
- Understand the migration shape (the `Nuke.*` shim under `src/Shims/`, the vendored `vs-solutionpersistence` fork).
- Suggest *where* a new feature should live, not just *how* to write it.

## Action
- [ ] Decide format and location — a dedicated `docs/architecture-for-agents.md`, or a restructured `docs/architecture.md` with explicit layer/module headers and stability annotations
- [ ] Capture current state:
- Module layering and dependency direction
- Public vs internal boundaries (especially the `internal`-only v11 middleware)
- Shim strategy (`src/Shims/Nuke.Common`)
- Vendored fork strategy (`vendor/vs-solutionpersistence`)
- Source-generator surface (`Fallout.SourceGenerators` outputs)
- [ ] Capture future state:
- v12 plugin SDK shape (link the open RFCs)
- CD vision (v13)
- Anything else explicitly in flux
- [ ] Cross-link from AGENTS.md so it's discoverable
- [ ] Add maintenance discipline — this doc decays fast; add a "last refreshed" date and an owner
- [ ] Optional: structure parts as YAML/JSON frontmatter or a sidecar file if it helps tools parse the module map

## Coordinate with
- AGENTS.md adoption issue — this doc gets linked from there
- Existing `docs/architecture.md` and `docs/rebrand-plan.md` — decide whether to merge, supersede, or cross-reference

## Done when
- A dedicated architecture brief for agents exists and is linked from `AGENTS.md`
- Current and future state both covered to the depth agents need
- A maintenance discipline (refresh date, owner) is in place