[idea] Let plugins draw the sidebar, with clickable rows

[putnap]

idea / problem

I run a dozen agents across a few spaces. For each one I want to see its pull request and its Linear issue in the sidebar, and click them. I got part of the way with a local plugin that writes extra rows under each space from workspace metadata tokens:

● js · #9488
  ○ Sync design analysis
  └─ js-connect-ingest-perf · ENG-4318 · #9745
     ▸ ◐ Load test monitoring          ← focused
       ✓ Connect ingest perf bench
● dotfiles
  ◐ Terminal agent stack

What a plugin can't do today:

• Clicking an agent row, the PR number or the issue key focuses the space. I open the PR and the issue with keybindings instead.
• The focus marker is a glyph a hook redraws after each focus change, so it lags. herdr's own highlight stops at the space.
• Navigate mode only steps over spaces, so the rows can't be reached from the keyboard.
• Tokens set text colour, bold and dim, with no backgrounds.
• Token values are trimmed, so I indent with a Braille blank, and each row needs its own token.

requested change

A way for a plugin to provide the sidebar's rows: each piece of text with a style and something to do on click (focus a pane or space, open a URL, run a plugin action), while herdr keeps drawing the focus highlight and handling the keyboard.

A smaller step would already help: clickable links inside sidebar tokens, like the links in panes. That alone makes the PR and the issue clickable.

Related: #5759, a plugin-added sidebar section.

why you want this

Desktop agent apps show each agent's PR and issue and let you jump to them. I'd rather stay in the terminal, and herdr is the closest I've found. With plugin-drawn rows, the tree above, a PR dashboard or a per-customer grouping could all be plugins instead of feature requests. I'm happy to try a preview build with my plugin.