`cmdStateAdvancePlan` expected separate `Current Plan` and
`Total Plans in Phase` fields, but the current STATE.md template
uses a single compound field: `Plan: X of Y in current phase`.
Now tries legacy separate fields first, then falls back to parsing
the compound format. Preserves the compound format when writing back
(replaces only the plan number). Also handles `Last activity`
(lowercase) field name from current template.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>