feat(tui): per-sub-agent context accounting in the TUI

[aheritier]

Summary

Surface each agent / sub-session's context usage (% of window) in the TUI. This is a multi-agent differentiator — no competitor exposes per-sub-agent context accounting.

Motivation

The plumbing already exists: TokenUsageEvent carries per-session/per-agent usage incl. ContextLimit, and the sidebar tracks a sessionUsage map. It just isn't surfaced per agent.

Proposed behavior

• Show per-agent / per-sub-session context % in the sidebar agent list and in the agent-details dialog.
• Include background agents (pkg/tools/builtin/agent).

Implementation notes (starting points)

• pkg/tui/components/sidebar/sidebar.go (+ layout.go)
• pkg/tui/dialog/agent_details.go

Complexity: M.

Background: shared plan context-management-gap-analysis (Phase 3, item 7).

Dependencies / Relationships

• Blocks: feat(runtime): cross-agent compaction orchestration (explicit, user-triggered) #3439 (cross-agent orchestration builds on per-sub-agent accounting)
• Related: feat(tui): add /context command with a categorized context-window breakdown #3432 (reuses breakdown computation), feat(tui): add warning/compacting states to the context-usage gauge #3434 (reuses the gauge)