fix(cli): initialize ctx_len before compact banner path

[helix4u]

What does this PR do?

Fixes a CLI startup crash in the compact banner path.

When Hermes starts in compact mode, show_banner() could reference ctx_len before it was initialized. This caused an UnboundLocalError during startup instead of showing the banner.

This change initializes ctx_len before the compact/non-compact branch so both paths are safe.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• Initialized ctx_len before the compact banner branch in cli.py
• Added a regression test in tests/test_cli_context_warning.py covering compact mode with unknown context length

How to Test

1. Start hermes in a narrow terminal or with compact mode enabled
2. Verify the CLI starts normally
3. Confirm the banner renders instead of crashing with UnboundLocalError

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes
• I've tested on my platform: Linux

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Test Notes

Targeted tests for this change pass:

• pytest tests/test_cli_context_warning.py -q
• pytest tests/hermes_cli/test_banner.py -q

[teknium1]

Fixed in PR #4411, which cherry-picked the implementation from #4334. Thanks for the contribution @helix4u!