fix: make config set examples use placeholder syntax

[teknium1]

Summary

• cherry-pick stablegenius49's placeholder syntax fix from fix: make config set examples use placeholder syntax #957 onto current main
• update the remaining config fallback help text to use <key> <value> too
• add regression tests for setup summary, config usage, show config output, and fallback help output

Testing

• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m pytest tests/hermes_cli/test_placeholder_usage.py tests/hermes_cli/test_set_config_value.py tests/hermes_cli/test_config.py -n0 -q
• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m pytest tests/ -n0 -q
• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m hermes_cli.main config set
• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m hermes_cli.main config show

Closes #926

Credit to @stablegenius49 for the original fix in #957.