Refresh daemon docs and mark web UI experimental

internal/daemon/doc.go and ARCHITECTURE.md were written before the
subpackage extractions and still referenced old structure (in-progress
phrasing, missing opstate/dmsnap/fcproc/imagemgr/session/workspace,
mentions of opRegistry by its old name). Both now describe the current
shape: composition root + six leaf subpackages, lock ordering rooted
at vmLocks[id], and the one intra-package dependency (workspace →
session for ShellQuote + FormatStepError).

README.md and AGENTS.md mark the local web UI as experimental. It is
still enabled by default at 127.0.0.1:7777, but the docs now state
plainly that its surface is not stable or hardened and not intended for
anything beyond single-user localhost use. AGENTS.md also points at
ARCHITECTURE.md for the subpackage layout.

No code changes; tests still green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -5,7 +5,8 @@ Always run `make build` before commit.
 ## Project Structure
 
 - `cmd/banger` and `cmd/bangerd` are the main user entrypoints.
-- `internal/` contains the daemon, CLI, RPC, storage, Firecracker integration, guest helpers, and web UI.
+- `internal/` contains the daemon, CLI, RPC, storage, Firecracker integration, guest helpers, and the experimental web UI.
+- `internal/daemon/` is the composition root; pure helpers live in its subpackages (`opstate`, `dmsnap`, `fcproc`, `imagemgr`, `session`, `workspace`). See `internal/daemon/ARCHITECTURE.md`.
 - `scripts/` contains explicit manual helper workflows for rootfs and kernel preparation.
 - `build/bin/` is the canonical source-checkout build output.
 - `build/manual/` is the canonical source-checkout location for manual rootfs/kernel artifacts.