refactor: modularize auth transport and markdown

- Split auth into focused package modules for agent keys, file keys,
  signing, and post-quantum transport helpers while keeping the public
  browser_cli.auth import surface intact.
- Move transport encoding internals into a package with separate codec and
  binary-hoisting helpers, preserving browser_cli.transport compatibility.
- Extract remote TCP auth/socket helpers and serve challenge setup out of the
  runtime paths to make connection handling easier to reason about.
- Move the extension markdown extractor into a dedicated content/markdown
  folder with separate root selection, code normalization, renderer, and utils.
- Centralize CLI Rich rendering helpers for tab/window tree and table output,
  and add rendering tests for the shared builders.
- Remove local typing ignores in SDK/decorator/script plumbing and bump the
  package and extension version to 0.15.3.

browser_cli/auth/signing.py

@@ -0,0 +1,42 @@
+"""Canonical browser-cli auth payload signing and verification."""
+from __future__ import annotations
+
+import hashlib
+import json
+
+from cryptography.exceptions import InvalidSignature
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey, Ed25519PublicKey
+
+from browser_cli.auth.agent import AgentKey, agent_sign_raw
+
+def canonical_payload(msg: dict) -> bytes:
+  """Deterministic JSON encoding of msg without auth protocol fields."""
+  return json.dumps(
+    {key: value for key, value in msg.items() if key not in {"pubkey", "sig", "pq_kex"}},
+    sort_keys=True,
+    separators=(",", ":"),
+  ).encode("utf-8")
+
+def auth_message(nonce: bytes, msg: dict, pq_shared_secret: bytes | None = None) -> bytes:
+  """Bytes signed for auth; optionally binds a post-quantum KEX secret."""
+  data = nonce + hashlib.sha256(canonical_payload(msg)).digest()
+  if pq_shared_secret is not None:
+    data += hashlib.sha256(b"browser-cli ml-kem-768 v1" + pq_shared_secret).digest()
+  return data
+
+def sign(key: Ed25519PrivateKey | AgentKey, nonce: bytes, msg: dict, pq_shared_secret: bytes | None = None) -> bytes:
+  """Sign nonce + payload hash, optionally bound to an ML-KEM shared secret."""
+  data = auth_message(nonce, msg, pq_shared_secret)
+  if isinstance(key, AgentKey):
+    return agent_sign_raw(key, data)
+  return key.sign(data)
+
+def verify(pub_hex: str, nonce: bytes, msg: dict, sig_hex: str, pq_shared_secret: bytes | None = None) -> bool:
+  """Return True if sig_hex is a valid signature over the canonical payload/auth secret."""
+  try:
+    pub_bytes = bytes.fromhex(pub_hex)
+    pub_key = Ed25519PublicKey.from_public_bytes(pub_bytes)
+    pub_key.verify(bytes.fromhex(sig_hex), auth_message(nonce, msg, pq_shared_secret))
+    return True
+  except (InvalidSignature, ValueError):
+    return False