fix(compression): add explicit auxiliary context_length config

[zkytech]

Summary

• add an explicit auxiliary.compression.context_length config key to the default config
• honor that override during compression feasibility checks before metadata probing/fallbacks
• document the new key in cli-config.yaml.example and cover it with a regression test

Closes #8810.

Context

This PR keeps the scope intentionally narrow: it only adds the explicit auxiliary compression context override requested in #8810.

I checked related work before opening this PR:

• fix: unify context length overrides for compression runtimes #8786 is a broader runtime override unification PR, but it does not add the explicit auxiliary.compression.context_length config surface requested by Feature: add explicit auxiliary.compression.context_length config #8810
• fix(agent): honor custom provider context in compression feasibility check #8721 targets compression feasibility and custom-provider context handling, but is broader/different in scope
• fix(compression): honor auxiliary context_length override #8667 was a closely related earlier PR that is now closed, so this PR reintroduces the explicit config key in a current, minimal form

Changes

• run_agent.py: read auxiliary.compression.context_length and forward it to get_model_context_length(..., config_context_length=...) during _check_compression_model_feasibility()
• hermes_cli/config.py: add auxiliary.compression.context_length to DEFAULT_CONFIG
• cli-config.yaml.example: document the new config key for custom OpenAI-compatible endpoints
• tests/run_agent/test_compression_feasibility.py: add regression coverage that the explicit override is forwarded and used by the warning path

Testing

• source venv/bin/activate && python -m pytest tests/run_agent/test_compression_feasibility.py -q

Contribution Guide Notes

• reviewed CONTRIBUTING.md before submitting
• checked for related issues/PRs first and linked them here
• kept the patch scoped to the requested config addition

[zkytech]

Closing this PR because already includes this functionality.