docs: rewrite README as a landing page

[jothimani-rajendran]

Summary

• Rewrites README.md as a landing page: centered header (chock logo — this repo has none of
  its own, name, one-line pitch, existing badges with Generated-only first), the chock demo
  GIF, a plain-language problem paragraph, then Install / What you get / Generated from
  chock-catalog / family table / License — per the shared README standard (org-plan #188 /
  epic #183).
• Keeps the VS Code / Copilot CLI install instructions exactly as they were.
• Keeps the enforce/advise wording as already stated in this repo's own README and
  PLUGINS.md: guard policies are "session-enforced where the host honours it", never an
  unqualified enforcement claim.
• No generated content touched: README.md is this repository's one hand-written file, per
  its own "Trust" section. chock marketplace build --dist . --check (framework pinned at
  v0.7.0, matching chock-claude-plugins' own verification steps) passes unchanged before
  and after.

Test plan

• git status --porcelain shows only README.md changed.
• chock marketplace build --dist . --check → Marketplace index matches the plugin tree (21 plugins).
• Word count: 499 (target 300–500).

🤖 Generated with Claude Code

[jothimani-rajendran]

The verify (Generated-only) check is failing here, but it isn't this PR's diff: chock marketplace build --dist . --check reports differs: PLUGINS.md because generated-only.yml installs the framework from open-coder-ai/chock's unpinned main (now 0.8.0), which has drifted from the v0.7.0 this tree was published against. The same check is already red on main at this PR's exact base commit (516ad59, the merge of #5) — see run 34175133625 — confirming it's a pre-existing, base-branch-wide break, not something this README-only change introduced.

A fix already exists open: #4, "ci: derive the framework ref from the catalog instead of a typed default." I'm not porting it into this branch myself — my scope here is README.md only (per this task's constraints, .github/workflows/* and other generated/infrastructure files are out of bounds for this worker), and #4 already addresses the root cause at the source. Once #4 (or an equivalent fix) merges to main, this branch should go green on the next push or rebase.

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