feat: restore html reports and mermaid specs

[satyaborg]

Summary

• Restore HTML run reports as the default, with Markdown still available via --report-format and --markdown.
• Bring back Mermaid spec diagrams and the bundled sibling HTML renderer.
• Cover renderer, installer, default HTML, and explicit Markdown report flows in tests.

Test plan

• bash scripts/devloop_test.sh
• git diff --check
• bash -n skills/devloop-spec/scripts/render.sh

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	devloop	a8d6f5f	Commit Preview URL Branch Preview URL	Jul 02 2026, 05:33 AM