Rewrite project and server documentation

[lobbystack]

Summary

Documentation-only change; no code is touched.

• README.md: rewritten as a public project page (features, Try Noura setup, storage overview with diagram, repository structure, contributing, support, license).
• docs/README.md (new): task-oriented index of guides and references.
• apps/server/OPERATIONS.md: reorganized into a runbook — deployment prep, S3 attachment storage, explicit limits, file-relay and realtime load/soak probes, and the automated offline restore rehearsal. Dated one-off local measurements and the open release-gates list are removed.
• apps/server/README.md, docs/architecture/sync-server.md: trimmed and refocused on setup and contracts.
• AGENTS.md: apps/server described as the existing experimental sync service; mentions apps/server-web.
• SECURITY.md: reflowed wrapped lines.
• Minor wording/link updates in ai-runtime, plugin-runtime, threat-model, local-alpha-acceptance, and workspace-format specs.
• Removed obsolete docs/architecture/ai-runtime-spike.md.

Reviewer notes

• Check that links in the new README and docs index resolve (e.g. THIRD_PARTY_NOTICES.md, docs/provenance/README.md, anchor links into v1.md and plugin-runtime.md).
• The removed release-gates list in OPERATIONS.md was the only place tracking unfinished work there — confirm it lives elsewhere.

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