docs: move SECURITY.md to .github/ and SPEEDRUN.md to docs/

Trims the repo root listing so the README is reached with less scrolling.
Only these two were movable; the other five root .md files are load-bearing
and stay put:

- README.md / README.zh-CN.md — the landing page and the language-switcher
  entry point
- CLAUDE.md — Claude Code loads project instructions from the ROOT path;
  moving it silently breaks every future session in this repo
- AGENTS.md — the agent-convention file Codex reads from the root and injects
  as context (see the comments in session-routes.ts)
- CHANGELOG.md — the changesets default writer emits it next to package.json,
  so moving it breaks `npm run version-packages`

GitHub officially resolves .github/SECURITY.md, so the Security policy tab
keeps working. Inbound links updated in both READMEs, CLAUDE.md and
docs/versioning-policy.md. CHANGELOG.md also names SECURITY.md but is left
alone: it is a historical record, not a live reference.

The move broke a link the other direction too: SECURITY.md pointed at
docs/security-architecture.md, which from .github/ resolved to
.github/docs/... — repointed to ../docs/. All relative links in the six
touched files verified resolving (62 links, 0 broken).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/versioning-policy.md

@@ -75,5 +75,5 @@ allowance. The commitments above take effect at `1.0.0`.
 ## See also
 
 - `CLAUDE.md` — the COM release workflow (changesets, version bump, deploy)
-- `SECURITY.md` — security reporting and the supported-version policy
+- `.github/SECURITY.md` — security reporting and the supported-version policy
 - `docs/security-architecture.md` — the full trust model