feat(ms-ai-architect): replace playground v2 with v3 + docs update

Step 17 (Wave 5, final). Closes the v3 playground delivery (5-session run,
17 commits total).

Pre-flight tests verified passing before deletion:
- bash tests/validate-plugin.sh -> 215/215 PASS
- bash tests/run-e2e.sh --playground -> 240/240 PASS (170 + 70)

Changes:
- DELETE playground/ms-ai-architect-v3.html
- MOVE v3 content to playground/ms-ai-architect-playground.html (3867 lines).
  Replaces the deleted v2 file at the same canonical path so external
  references continue to resolve.
- UPDATE tests/test-playground-v3.sh + tests/test-playground-parsers.sh
  to point at the renamed canonical file.
- UPDATE plugin README.md (## Playground (v3) section): describes the
  4-surface decision-builder + report-viewer architecture, persistent state
  model, 17 report renderers, theme toggle, and the validation matrix.
- UPDATE plugin CLAUDE.md: replaces v2 5-step pipeline section with v3
  architecture overview. Marks docs/playground-v2-spec.md as historical-only
  (no longer the contract); points at .claude/projects/2026-05-03-playground
  -v3-architecture/ for v3 spec.
- UPDATE root README.md: marketplace listing for ms-ai-architect now
  describes v3 architecture (4 surfaces, persistence, 17 renderers, theme,
  240-test validation) and references the test command.

Verify (post-rename):
- ! test -f playground/ms-ai-architect-v3.html: pass
- test -f playground/ms-ai-architect-playground.html (>3000 lines): pass
- grep -q "v3" in plugin README + plugin CLAUDE.md + root README: pass
- bash tests/validate-plugin.sh: exit 0 (215/215)
- bash tests/run-e2e.sh --playground: exit 0 (240/240)

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