_normalize_approval_mode() previously accepted any string, so an unknown
value like 'auto' fell through every downstream mode check (off/smart) and
silently behaved like manual with no signal. Validate against the known
modes (manual/smart/off), emit a warning for anything else, and default to
manual to match the config default and the rest of the function.
Bug 1 from the original PR (/approve & /deny bypassing the running-agent
guard) already landed on main independently, so only the mode-validation
fix is salvaged here.
Fixes#4261
Co-authored-by: Hermes Agent <agent@nousresearch.com>
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