Replace README placeholder screenshots with real captures

[charneykaye]

The README and docs still ship SVG placeholder screenshots. Milestone 1 polish (ROADMAP.md → "Remaining polish — before Milestone 1 tag") calls for real captures of an actual gameplay session.

Scope

• Capture the six screens listed in docs/screenshots.md (Home, Scenario Library, Conversation, Debrief, Creator Workbench, Model Manager) from a real playthrough with a real local model
• Replace the hero demo placeholder (docs/assets/demo-placeholder.svg) with an animated GIF or short video of a mid-session conversation
• Preserve the alt-text quality documented in docs/screenshots.md

Acceptance

• No *-placeholder.svg or placeholder screenshot references remain in README.md or docs/
• Images show a real session (visible state meters, real NPC output), not mockups
• docs/screenshots.md checklist items all checked

Requires a stable real-model playthrough on real hardware — human capture; see the replacement checklist in docs/screenshots.md.