Closes #3434

The always-on context gauge escalates its color (normal, warning, critical) as usage approaches the agent's configured compaction threshold, and reads "compacting..." while a compaction runs. Applies to both the main TUI sidebar and the lean TUI status line.

Issue expectations mapping

Flow

runtime loop
  SessionUsage(sess, contextLimit, a.CompactionThreshold())
    -> TokenUsageEvent.Usage.compaction_threshold
         -> sidebar: warning/error style on "(N%)" and roster percentages
         -> leantui: RenderBar(pct, threshold) color escalation

doCompact
  SessionCompaction(started) ........ SessionCompactionCompleted(outcome)
    -> sidebar: "(compacting...)" replaces the percent, spinner stays active
    -> leantui: "compacting..." replaces the percent, token counts stay visible

Behavior details

• Escalation bands are relative to the threshold (0.75x and 0.95x of it), so a custom compaction_threshold keeps a proportional visual runway; thresholds outside (0, 1] fall back to compaction.DefaultThreshold, mirroring compaction.ShouldCompact.
• The lean TUI bar previously used hardcoded 0.6/0.85 breakpoints; at the default threshold the new bands land nearby (0.675/0.855).
• Usage.CompactionThreshold is 0 (omitted from JSON) when unknown, so events from older servers keep working; consumers fall back to the default.
• ESC cancel clears the sidebar compacting state; the lean TUI clears it on conversation reset.
• SessionUsage gains an optional variadic threshold parameter, following the AgentInfo(..., contextLimit ...int64) backward-compatibility precedent in the same file.

Tests

Validation: build, golangci-lint plus repo lint cops, go mod tidy --diff clean, full test suite green in touched packages (remaining repo failures are pre-existing sandbox network SSRF-guard tests, verified on a pristine checkout).