Add Agents tab for per-agent token tracking

[BubbleBuffer]

Adds a new Agents page that breaks down token usage by agent (the agent field from OpenCode usage events). Previously the tool only had Overview, Models, and Providers pages.

What's new

• Agents tab in the TUI header (navigate with Tab / Left / Right arrows)
• Per-agent chart showing token distribution across agents
• Agent list with keyboard navigation (j/k) and fuzzy search (f)
• Detail panel for focused agent: cost, sessions, tokens, messages, input→output, cache, rate (p50 OPS)
• Model sub-breakdown per agent: which models each agent used with tokens, sessions, and cost
• Share card summary includes agent info

Files changed

File	Change
src/analytics/agent_stats.rs	New — AgentUsageRow, AgentModelBreakdown, build_agent_chart(). Groups events by agent, builds per-model sub-breakdowns
src/ui/agents.rs	New — render_agents() TUI page with chart, list, detail panel, model sub-breakdown, search
src/analytics/model_stats.rs	Made UsageAccumulator, build_chart_for_models, median public
src/ui/models.rs	Made SearchItem, render_search_overlay, layout_rows public
src/analytics/mod.rs	Added agent_stats module, agents + agent_chart fields
src/ui/mod.rs	Added agents module
src/ui/app.rs	Wired Agents page into tab navigation, search, and share card

Verification

• cargo build: 0 warnings
• cargo test: 39/39 passed
• cargo fmt: clean