feat: add persistent CLI status bar and usage details (#1522)

Salvaged from PR #1104 by kshitijk4poor. Closes #683.

Adds a persistent status bar to the CLI showing model name, context
window usage with visual bar, estimated cost, and session duration.
Responsive layout degrades gracefully for narrow terminals.

Changes:
- agent/usage_pricing.py: shared pricing table, cost estimation with
  Decimal arithmetic, duration/token formatting helpers
- agent/insights.py: refactored to reuse usage_pricing (eliminates
  duplicate pricing table and formatting logic)
- cli.py: status bar with FormattedTextControl fragments, color-coded
  context thresholds (green/yellow/orange/red), enhanced /usage with
  cost breakdown, 1Hz idle refresh for status bar updates
- tests/test_cli_status_bar.py: status bar snapshot, width collapsing,
  usage report with/without pricing, zero-priced model handling
- tests/test_insights.py: verify zero-priced providers show as unknown

Salvage fixes:
- Resolved conflict with voice status bar (both coexist in layout)
- Import _format_context_length from hermes_cli.banner (moved since PR)

Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>

tests/test_insights.py

@@ -206,6 +206,7 @@ class TestHasKnownPricing:
     def test_unknown_custom_model(self):
         assert _has_known_pricing("FP16_Hermes_4.5") is False
         assert _has_known_pricing("my-custom-model") is False
+        assert _has_known_pricing("glm-5") is False
         assert _has_known_pricing("") is False
         assert _has_known_pricing(None) is False