feat: show the agent's logo on terminal tabs

[vanvasten]

Summary

• A terminal tab running a detected agent (Claude Code, Codex, Copilot, Cursor, Gemini CLI, OpenCode) shows that agent's logo instead of the generic terminal glyph, in both the native and the SwiftUI tab strips. Logos stay in full colour on inactive tab bars.
• Logos are from Lobe Icons (MIT), attributed in THIRD_PARTY_LICENSES.md and Assets.xcassets/AgentIcons/ATTRIBUTIONS.md.
• Stacked on fix: fade tab strip edges when tabs overflow #433 (it touches the same tab-bar files). The base is that branch until fix: fade tab strip edges when tabs overflow #433 merges; then this rebases onto main.

Testing

• AgentActivityStateTests gains cases for the agent-to-icon mapping.
• A Debug xcodebuild compiled the Swift sources; the only local failure was the Ghostty resources copy script (no zig here). CI runs the tests.
• Logos not checked by eye in light and dark.

Demo Video

• Video URL or attachment: needed from the author (visual change).

Checklist

• Tested via CI and, for UI or behavior changes, a tagged build (./scripts/reload.sh --tag <name>)
• I added or updated tests for behavior changes
• I updated docs/changelog if needed
• I requested bot reviews after my latest commit
• All code review bot comments are resolved
• All human review comments are resolved