The great refactoring

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Align the repo with the marketplace/plugin model. Scope (issue #15): migrate docs/issues/ and docs/prd/ to Gitea; slim ROADMAP.md to a nav page; rewrite docs/spec/overview.md and docs/spec/architecture.md for the plugin model; update CONTEXT.md (drop file-based issue tracker, add LESSONS.md entry); archive stale notes; delete stale bin skills (to-issues, to-prd); clean install.sh dead code; triage and realign all ADRs.

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