[FEATURE]: Real-time token generation speed (tok/s) in TUI status bar, like Hermes

Describe the enhancement you want to request

I want opencode to show real-time token generation speed (tok/s, tokens per second) while a response is streaming, like DeepSeek Hermes agent does.

Hermes shows it directly in the status bar, e.g.:

deepseek-v4-flash | 12.5K/128K | 68% | 3m 42s | 15.2 t/s

Reference implementations:

• https://github.com/maxxqf-ai/hermes-status-bar-tps (adds XX.X t/s to Hermes status bar from completion_tokens / api_duration)
• https://github.com/NousResearch/hermes-agent/pull/44878 (per-call last_api_duration + last_output_tokens -> output_speed, exposed in CLI status bar / desktop status bar / gateway runtime footer behind display.show_output_speed)

Proposed behavior in opencode

1. TUI: live rolling tok/s while streaming (in the status line / session footer), plus final per-response average once complete (next to existing token counts).
2. Desktop / web: same t/s indicator for parity.
3. Optional session-wide average (e.g. via /stats or footer) for comparing models/providers.
4. Config toggle, e.g. display.show_output_speed (default off to avoid clutter), and hide when terminal width is too small.

Why

• Currently there is no way to gauge inference throughput per response or compare models/providers without external benchmarking.
• Very useful when switching between fast models (e.g. DeepSeek V4 Flash ~80-150 tok/s) and slower frontier models — speed directly affects interactive coding feel.
• Related open requests: #43857, #6096. This issue explicitly proposes the Hermes-style UX (live + per-call average + toggle) as the concrete design.

Alternatives considered

• Timing requests manually or benchmarking provider APIs directly — does not reflect real usage inside opencode.
• Only showing total tokens / cost after completion — no sense of live speed.