Split monolithic tai.py into tai_pkg package, keep tai-orig.py as original

tai.py (8643 lines) is now a thin facade over tai_pkg/ (33 feature modules, one class or feature per file). tai-orig.py preserves the pre-split single-file version byte for byte (sha256 verified against snapshot); import tai and ./tai.py behavior unchanged.

Equivalence proof, all run on this tree immediately pre-commit: differential battery tai-orig.py vs facade 3718 checks 0 failures (surface, pure fuzz, tool dispatch, store/audit, release ops, fake-HOME install roundtrip, CLI runs); test_tai.py 261 OK; test_seal.py 16 OK; --version/--help byte-identical; dir(tai) zero removals.

Fix included: opencode wire-protocol schema echo leaked into replies; opencode_parse now scans with JSONDecoder.raw_decode capped at 200 scans (13 OpencodeParseTests). Test count 261 = 237 HEAD + 8 pre-existing uncommitted ToolArgumentsTests (now committed with this change, disclosed) + 13 OpencodeParseTests + 3 EmbeddedStringsTests. Patch-target-only test migration (24 hunks), zero assertion changes. tai_pkg clean-regenerated after mid-verification tamper was found and reverted.

tai_pkg/compat.py

@@ -0,0 +1,24 @@
+"""Optional-dependency import guards (readline, fcntl, pty set)."""
+
+__all__ = ["readline", "fcntl", "pty", "select", "termios", "tty"]
+
+
+try:
+    import readline
+except ImportError:
+    readline = None
+
+
+try:
+    import fcntl
+except ImportError:
+    fcntl = None
+
+
+try:
+    import pty
+    import select
+    import termios
+    import tty
+except ImportError:
+    pty = select = termios = tty = None