Prepare repository for public release

- README: attract-mode stills + demo GIF as the hero row, status/warning
  callouts, metrics table, "Contents" index, doc-index table; simplified
  the two-agent workflow diagram (LR -> TD, fewer edges) for GitHub web
- Table-of-contents indexes added to internals.md, porting-notes.md,
  agentic-development.md, AUDIO-PLAN.md
- CI: add Windows/MSYS2 build job, minimal token (contents: read),
  persist-credentials: false, concurrency group; run on main + master.
  validate job regenerates the US ROM-asset symbol file only and configures
  ntsc-final (PAL/JP need a region ROM; no ROM is used in CI)
- .github: SECURITY.md, issue forms + chooser config, PR template,
  labels.yml, dependabot (Actions, monthly)
- Do not track the proprietary SGI IRIX IDO toolchain or SGIImageViewer.exe;
  document in SetupGuide.md / building.md how to obtain IDO for N64 ROM
  builds (the PC port does not need it)
- Stop tracking two committed *.pyc files; drop stale screenshots and a
  redundant .gitignore line; misc doc cross-reference fixes

Claude-Session: https://claude.ai/code/session_01GULf6knbsqfsnW39tMr9WT

.gitattributes

@@ -1,6 +1,8 @@
 * text=auto eol=lf
 *.png binary
 *.jpg binary
+*.gif binary
+*.mp4 binary
 *.z64 binary
 *.exe binary
 *.bin binary