Rewrite README's Architecture section: drop the `ptyc/` description
and `make ptyc-build` line, fix the broken `decisions/` link
(governance moved it), add pointers to current docs.
Banner docs/initial-plan.md as historical with a redirect to a new
docs/architecture.md that describes today's shape (single Flutter
package at the repo root; in-process IPC + dispatcher + subsystems;
PTY via Dart FFI posix_openpt + posix_spawn).
Also banner two ptyc-era forensic notes (docs/pty-proposition.md,
docs/macos-pty-problem.md) as historical so a contributor reading
them isn't misled into thinking the C helper still exists.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
