v3.20.0 - pforge self-update --verify + capabilities-drift CI + metrics refresh

v3.20.0 - pforge self-update --verify + capabilities-drift CI + metrics refresh

Adds composable --verify flag to pforge self-update that chains pforge check and pforge smith against the freshly-installed wrapper. New capabilities-drift CI workflow blocks drift between MCP discovery surface and published docs/capabilities.md. Refreshes _metrics.json and 26 user-facing documentation surfaces to current canonical counts (104 MCP tools, 21 reviewer agents, 4 reviewer skills, 30 manual chapters). Manual catch-up v3.12 -> v3.19 across 12 chapters.