Update README to reflect official rigctl/Hamlib support and Mermaid architecture

[Copilot]

README still described rig control as “planned” despite Hamlib-backed rigctl support being available, and the architecture was rendered from a static diagram asset. This updates the docs to match current capabilities and switches the architecture representation to Mermaid for in-repo readability/editability.

• Support status update

  • Replaced outdated wording that listed rigctld as pending.
  • Clarified current state: WSJT-X is supported and rig control via rigctl/Hamlib is officially supported; Ham Radio Deluxe remains planned.

• Architecture diagram migration

  • Replaced ![Architecture](architecture.svg) with an inline Mermaid flowchart.
  • Kept the same topology and links while aligning labels with current terminology (rigctl/Hamlib).

• Documentation consistency

  • Synchronized feature language and diagram labels so architecture and capability statements no longer diverge.

```mermaid
flowchart LR
    Application ---|websocket| agent
    agent ---|UDP| WSJTX
    agent ---|lib| rigctld[rigctl/Hamlib]

[xylo04]

@copilot

• Rebase on main, I thought I had already merged those changes but hadn't
• In the mermaid diagram, make sure the kel-agent node is highlighted.

[Copilot]

@copilot

• Rebase on main, I thought I had already merged those changes but hadn't
• In the mermaid diagram, make sure the kel-agent node is highlighted.

Done. The branch has been rebased onto the latest main, and the Mermaid diagram now highlights the kel-agent node in commit 567a998.

[xylo04]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved. I rebased and fixed the README conflict(s); the branch is now conflict-free in commit b9f4fbd.