bug(agents): subagent cost uses 2 decimals instead of the shared formatCost (header shows 3 below $1)

[vexny]

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Problem

The Agents widget card and the Agents view task header render subagent cost with exactly two decimals ($${task.cost.toFixed(2)}), while the shell bar / fullscreen header formats cost through the shared formatCost() helper, which shows three decimals below $1 (e.g. $0.090) and two at or above $1 (e.g. $12.50). A subagent that cost $0.09 shows $0.09 in the Agents UI while the same magnitude renders $0.090 in the header: the two surfaces drift on how they paint the same fact, and small costs lose precision.

Steps to reproduce

1. Start gentle-shell.
2. Run a subagent (subagent_run) that accumulates a cost below $1 (e.g. $0.09).
3. Observe the Agents card above the editor and the Agents view (alt+a) task header.
4. Compare with the shell bar / fullscreen header cost segment.

Expected and actual behavior

Expected:
Subagent cost uses the same formatCost() as the header: three decimals below $1, two at or above $1 ($0.090, $12.50).

Actual:
Subagent cost is hardcoded to two decimals ($0.09, $12.50), diverging from the header.

gentle-pi version

3.7.0 (npm gentle-pi)

Pi version

0.87.1

Operating system

Linux

Relevant logs or error output (optional)

Source: lib/agents-widget.ts (metaFields) and lib/agents-view.ts (taskHeader) hardcode $${task.cost.toFixed(2)}, while lib/shell-bar.ts exports the shared formatCost(total, subscription) already used by the header.