fix(shell): hide context usage until warning threshold

[binichallein]

Related Issue

Resolve #2291

Description

• Stop rendering the shell bottom toolbar context usage meter while usage is below the warning threshold.
• Keep the context meter visible once usage reaches 80%, where the user may need to act.
• Preserve right-side toast behavior so transient status messages still appear in the same toolbar area.

Verification

• RED: uv run pytest tests/ui_and_conv/test_prompt_tips.py -k "context or running_prompt_uses_shared_toolbar" -vv failed before the implementation because low context usage was still rendered.
• GREEN: uv run pytest tests/ui_and_conv/test_prompt_tips.py -k "context or running_prompt_uses_shared_toolbar" -vv
• uv run pytest tests/ui_and_conv/test_prompt_tips.py tests/core/test_status_formatting.py -vv
• uv run ruff check src/kimi_cli/ui/shell/prompt.py tests/ui_and_conv/test_prompt_tips.py
• uv run ruff format --check src/kimi_cli/ui/shell/prompt.py tests/ui_and_conv/test_prompt_tips.py
• uv run pyright src/kimi_cli/ui/shell/prompt.py tests/ui_and_conv/test_prompt_tips.py
• git diff --check

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog. (N/A: small UI behavior fix; no user docs/changelog generated.)
• I have run make gen-docs to update the user documentation. (N/A: no docs generation needed.)

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.