feat(tui): add warning/compacting states to the context-usage gauge

[aheritier]

Summary

Give the always-on context gauge visual warning states as usage approaches the compaction threshold, and show a "compacting…" state while compaction runs.

Motivation

The sidebar already renders "Context: N%" but with no color/warning cues. Users get no signal that they're near the limit or that compaction is happening.

Proposed behavior

• Color the gauge (e.g. normal → warning → critical) as it nears the configured compaction threshold.
• Show a "compacting…" indicator driven by the existing SessionCompactionEvent (pkg/runtime/event.go ~line 471).
• Apply to both the main TUI sidebar and the lean TUI status line.

Implementation notes (starting points)

• pkg/tui/components/sidebar/sidebar.go (gauge at ~560-566, 1109-1137)
• pkg/tui/styles/styles.go
• pkg/leantui/update.go

Complexity: S.

Background: shared plan context-management-gap-analysis (Phase 1, item 3).

Dependencies / Relationships

• Blocked by: feat(config): make the auto-compaction threshold configurable (compaction_threshold) #3433 (needs the configurable compaction threshold to color against)
• Related: feat(tui): per-sub-agent context accounting in the TUI #3438 (per-sub-agent accounting reuses the gauge)