fix(proxy,tooling): serialize+reconnect Caido client, actionable HTTPQL errors, sandbox tool guidance

Addresses the top recurring agent tool-call failures observed in telemetry:

- proxy: the shared Caido client had no locking or reconnect, so concurrent
  agent calls raced ("Transport is already connected") and a dead transport
  poisoned the rest of the run ("Connector is closed"/"Server disconnected").
  Add an asyncio lock + bounded reconnect in caido_api.call_with_client (sandbox
  path) and a scan-wide caido_lock in the run context that host-side proxy tools
  hold around every call. Deterministic errors are not retried.
- proxy: list_requests now returns Caido's exact parser message, echoes the
  offending query, and includes a corrected-syntax hint so agents self-correct
  instead of retrying a broken HTTPQL filter.
- shell/prompt: document that write_stdin requires a process started with
  tty=true; nudge toward writing Python to a file over deeply-nested one-liners;
  note the venv pre-installs common libs.
- agent-browser: distinguish daemon/connection failures (run doctor, don't loop)
  from malformed commands; invoke directly (no sh -c wrapper).
- containers: use POSIX '.' instead of the bashism 'source' in generated rc
  files (fixes 'sh: source: not found'); add file + xxd and pre-install
  requests/httpx/beautifulsoup4/lxml/pyjwt/cryptography in the sandbox venv.
- tests: cover proxy serialization/reconnect/no-retry and HTTPQL errors.

containers/Dockerfile

@@ -24,6 +24,7 @@ RUN apt-get update && \
     python3 python3-pip python3-dev python3-venv python3-setuptools \
     golang-go \
     net-tools dnsutils whois \
+    file xxd \
     jq parallel ripgrep grep \
     less man-db procps htop \
     iproute2 iputils-ping netcat-traditional \
@@ -192,6 +193,8 @@ RUN mkdir -p /workspace && chown -R pentester:pentester /workspace /app
 USER pentester
 RUN python3 -m venv /app/.venv && \
     /app/.venv/bin/pip install --no-cache-dir caido-sdk-client && \
+    /app/.venv/bin/pip install --no-cache-dir \
+        requests httpx beautifulsoup4 lxml pyjwt cryptography && \
     /app/.venv/bin/pip install --no-cache-dir -r /home/pentester/tools/jwt_tool/requirements.txt && \
     printf '%s\n' \
         '#!/bin/bash' \