chore: collapse the repo to the four product trees

[CREDO23]

Summary

• Drop the sunset client trees (surfsense_desktop, surfsense_browser_extension, surfsense_obsidian, surfsense_evals) so the working repo is the four products: desktop, API, self-host web, MCP.
• Add root AGENTS.md, a short CONTRIBUTING.md, and per-product READMEs for backend and web so a first PR only needs one tree.
• Trim leftover skills, old plan trees, and unused docker extras; keep plans/community-local.

This sits on current dev (including the 2.0 README). Architecture matches the earlier re-reorg work; the product code underneath is the newer dev snapshot.

Test plan

• Top-level listing is the four trees plus docker/, plans/community-local, AGENTS.md, CONTRIBUTING.md
• Root README.md is still the 2.0 desktop page (hero, Studio still, 18 October sunset date)
• CONTRIBUTING.md links to surfsense_local, surfsense_mcp, surfsense_backend, and surfsense_web READMEs
• New issue templates (bug.yml, feature.yml, docs.yml) render on GitHub
• Desktop, backend, and web each still have a setup path in their own README
• CI on this PR is green

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