fix onboarding docs to match current architecture (T-101)

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>

docs/macos-pty-problem.md

@@ -1,5 +1,11 @@
 # macOS PTY Problem — Diagnosis Complete
 
+> **⚠ HISTORICAL — `ptyc` is retired.** PTY spawning moved to
+> Dart FFI `posix_openpt()` + `posix_spawn()` (D-5 amendments,
+> T-96). The race described below was specific to the old C helper's
+> SCM_RIGHTS hand-off and no longer applies.
+
+
 ## Status: Root cause found
 
 The PTY master fd is valid (`isatty=1`), SCM_RIGHTS transfer is correct, all struct layouts are correct. The problem is **timing**: the reader isolate starts too late and the shell has already exited by the time `read()` is called on the master fd. macOS returns EOF (n=0) immediately when the slave side is closed — unlike Linux which buffers data.