feat: add Session Cost (with subagents) widget

[lala74]

Summary

Adds a Session Cost (with subagents) widget (session-cost-total) that shows the full session cost in USD: Claude Code's cost.total_cost_usd plus the cost of all subagents (Task/Agent tool) spawned this session.

Renders as Total Cost: $3.55 (or $3.55 raw). Pairs naturally with the existing session-cost widget (main-only) on the same line.

Relates to #474.

Note: this PR previously contained an agent-count widget; it's been replaced — the cost total is the actually-useful piece.

Problem

cost.total_cost_usd from the status line stdin covers only the main transcript. Subagent runs are stored in separate <session>/subagents/agent-*.jsonl transcripts and are excluded from that number, so the displayed cost stays flat while subagents burn tokens. Heavy parallel-agent sessions show a misleadingly low cost.

How it works

getSubagentCostUsd() reuses the subagent discovery already used by the speed-metrics widgets (referenced agentIds ∩ on-disk agent-*.jsonl), then prices each transcript from its per-call token usage:

• model-pricing.ts — per-MTok price table by model family (Opus 5/25, Sonnet 3/15, Haiku 1/5, Fable 10/50), cache write at 1.25× input, cache read at 0.1× input; Opus-tier default for unknown models.
• Same streaming stop_reason dedup as getTokenMetrics, so per-call usage isn't double-counted.

The widget total = stdin total_cost_usd (main, authoritative) + computed subagent cost. Computed only when the widget is present on a line (same gating as speed/clock widgets).

Changes

• src/utils/model-pricing.ts (new) — price table + costForUsage
• getSubagentCostUsd() in src/utils/jsonl-metrics.ts (+ barrel re-export)
• RenderContext.subagentCostUsd; precomputed in src/ccstatusline.ts only when the widget is used
• SessionCostTotalWidget (src/widgets/SessionCostTotal.ts) registered in widgets index + manifest
• model added to the transcript message type (TokenMetrics.ts)
• tests: model-pricing.test.ts + subagent-cost cases in jsonl-metrics.test.ts

Testing

• bun run lint — clean
• bun test — 1596 pass (7 new)
• Manual e2e against real sessions: with stdin total_cost_usd: 2.00, a session with one subagent renders Total Cost: $2.46; a session with no subagents renders Total Cost: $2.00.