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/docker-entrypoint.sh

@@ -91,10 +91,13 @@ http_proxy=http://127.0.0.1:${CAIDO_PORT}
 https_proxy=http://127.0.0.1:${CAIDO_PORT}
 EOF
 
-echo "source /etc/profile.d/proxy.sh" >> ~/.bashrc
-echo "source /etc/profile.d/proxy.sh" >> ~/.zshrc
+# Use POSIX `.` (not the bashism `source`) so these lines are safe when the rc
+# files are read by a POSIX shell (e.g. `sh -lc`), which otherwise fails with
+# "source: not found". `.` is understood by bash, zsh, and dash alike.
+echo ". /etc/profile.d/proxy.sh" >> ~/.bashrc
+echo ". /etc/profile.d/proxy.sh" >> ~/.zshrc
 
-source /etc/profile.d/proxy.sh
+. /etc/profile.d/proxy.sh
 
 echo "✅ System-wide proxy configuration complete"