fix(hub): close seven bugs found in the refactor review

The PermissionRequest hook posted asks cut to the hub's field limits, so a signature covered less than Claude Code would run; it now posts nothing it cannot sign in full and leaves the decision to Claude Code with a reason on stderr. fields.seconds names the field the caller used in its error. Serving a stored page maps an OSError to 500 instead of a dropped connection. The natural-language line marks the register as seen before it builds the inbox, like the page does. do_POST runs every header guard (host, origin, client, content type, MCP) before it reads the body, so a refused request never costs its upload. The runner's reader closes stdout at EOF, because a close from the executing thread blocked on the buffer lock and could SIGPIPE a command still running. The page title counts only inbox items that are not snoozed.

Committed on the operator's word despite the gate: cost and time are estimates under 98 % other load and include eight new test cases.

🎯 Fix: 88 %
💥 Break: 3 %
💸 Cost: 106 %
⏱️ Time: 113 %
🛡️ Security: 100 %
🏆 Score: 71
🚦 Gate: ⛔ fix 88 % < 90; cost 106 % > 100, time 113 % > 100: ask the operator before the commit

```
/home/kevinveenbirkenbach/Repositories/github.com/kevinveenbirkenbach/skills/.worktrees/feature/kvb-hub
```

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hub/kvb_hub/exchange.py

@@ -97,7 +97,7 @@ class Exchange(BaseHTTPRequestHandler):
     def client(self) -> bool:
         return self.headers.get_all(GUARD) == ["1"] and self.headers.get_all("Origin") is None
 
-    def body(self, limit: int) -> bytes | None:
+    def body_length(self, limit: int) -> int | None:
         length = self.headers.get("Content-Length")
         if length is None or not LENGTH.fullmatch(length):
             self.send_text(HTTPStatus.LENGTH_REQUIRED, "Content-Length required")
@@ -105,4 +105,4 @@ class Exchange(BaseHTTPRequestHandler):
         if int(length) > limit:
             self.send_text(HTTPStatus.REQUEST_ENTITY_TOO_LARGE, "body too large")
             return None
-        return self.rfile.read(int(length))
+        return int(length)