feat: add debug tool command to call tools directly

[dgageot]

Debugging a tool often means wiring up a full agent config just to see
what one specific tool returns for a given set of parameters. This adds
a debug tool command that calls a tool directly, outside of any LLM
loop, so misbehaving tools can be reproduced and inspected quickly.

docker agent debug tool <config> <tool> [JSON params] loads the normal
toolsets from the given config, lists and invokes deferred tools as
needed, and runs the named tool with the given JSON parameters
(defaulting to {} when omitted). -a/--agent selects which agent's
toolset to use, and --json switches from a trimmed, readable summary
to the full raw result. The command surfaces warnings and result errors
and makes sure resources are cleaned up afterwards.

The follow-up commit hardens this against two gaps found during review:
the command now stops acting once its context is canceled instead of
pressing on with a dead client, and it rejects attempts to launch
built-in background jobs (including from code-mode and deferred tools)
since those are long-running and don't make sense in a one-shot debug
invocation. Both commits are covered by tests, including MCP structured
and media content round-trips, tool filtering, canceled filesystem
writes, and the various recall code paths; docs were updated to
describe the new command.