Partners page + top-level page system

Added the Partners page and turned top-level pages into a first-class, drop-in concept.

• Partners page (partners.html), built from the partner strategy: positions Risk Mandate as the risk-acceptance and accountability layer that sits on top of the agent-detection ecosystem — not a competitor to it. Covers the two areas of operation (partner enablement and marketplace referral), a worked example that deepens one detection finding into a fundable business case, the recommendation format, and the open-source / no-lock-in integration story.
• Top-level page system. Standalone pages now live under src/pages/* and are auto-discovered by the build — each becomes <name>.html. A shared rm-page-nav renders links to all of them from an inlined set, so the header page nav stays in sync everywhere. Adding the next page (the Library) is just a folder.
• The header "Dev" link is now part of that shared page nav, alongside "Partners".
• The release marker (version) is now tracked independently of the default design version the host loads (versions.json latest), since adding a page doesn't change either site design.