Optional per-kind icon override for agent labels

[lunetics]

Herdr already knows each pane's agent kind, and state_icon gives me the state as a generic symbol (●, ○, ×, ◐). What I can't get is a per-vendor glyph, so six reviewer panes read as six short lowercase words that blur together at a glance.

This already works, but only by hand: pane report-metadata --display-agent with a Private Use Area glyph survives the sanitiser, which filters control characters only (src/app/api/panes.rs:1612-1620), so the icon renders. The cost is that every pane needs an external script to set it, including panes where someone just ran codex in a shell and the detector already identified it correctly.

What I'd suggest is a lookup table, not bundled icons and not a bundled font:

[ui.agent_icons]
codex = "\U000F40D0"
claude = "\U000F4036"

The agent token would prefix the glyph when the kind has an entry. No font ships with herdr, no new render primitive, and anyone who sets nothing sees what they see today. Ready-made glyph sets exist for people who want one: lobe-icons-font covers 309 vendor logos in Plane 15, which does not collide with Nerd Font ranges.

I'm aware of the direction in #955 ("i don't want to bring more primitives but it should be extendable by anyone"). This is meant in that spirit: a lookup table over a token that already exists, so the detector's own knowledge of the kind is enough and each user doesn't re-derive it in a hook.

[vukvukovich]

Ran into the same need running several agent kinds side by side. Worth noting nerd fonts already ship brand glyphs (cod-claude ec82, cod-openai ec81, cod-copilot ec1e), so the lookup-table approach covers the common kinds with no bundled font. Opened #2839 proposing user-configurable state glyphs - the two could share the same icon-overrides home.