Update README to highlight and document the new developer agent
and improve clarity around available agents and coordinator examples.

- Rename "Run an agent" to "Run a specialized agent for codebase
  exploration" and update example command to use the new
  developer agent invocation.
- Add local shell wrapper example for running the developer agent.
- Rename "Current Agents" to "Featured Agents" and reorder the
  agent list to surface the new developer agent first.
- Provide a short showcase section for the developer agent that lists
  its local tools (file discovery, structure exploration, code
  outlining, ripgrep pattern search) and references its
  implementation path.
- Reformat travel coordinator section: clarify orchestration example,
  restyle step descriptions with bold headings, and point to the
  travel coordinator implementation path.
- Minor wording and formatting tweaks for consistency (list labels,
  command descriptions, and log/access notes).

These changes make the README more useful for developers by calling
out the new developer agent, documenting its capabilities, and
making examples easier to follow.