Show args and details for generic (extension) tools

[dnouri]

Previously, generic tools (e.g. subagent, or any extension tool) showed only the tool name in the header and discarded the details payload. Built-in tools like bash ($ ls -la) and edit (edit foo.txt) had rich headers — generic tools were second-class citizens.

Now:

• Headers show the tool's JSON args — compact when they fit within fill-column, pretty-printed otherwise
• Output appends non-nil details as formatted JSON below the content, with the same collapse/expand behavior as other tools

Built-in tools (bash, read, write, edit) are unchanged.

Example

A subagent call now renders as:

subagent { "agent": "worker", "task": "Search for weather" }
Task completed

**Details**
{
  "mode": "single",
  "exitCode": 0
}

instead of just:

subagent
Task completed