feat(specs): add editable product proposals

Summary

• Add Specs as a first-class product proposal workflow with list, create, show, edit, comment, and revision-history LiveViews.
• Store specs, comments, and immutable spec revisions with optional feature-request source references, member-only create/edit permissions, authenticated comments, and optimistic revisions for local editing.
• Render proposal bodies and comments as safe Markdown, show Specs in a Noora table with URL-backed status filters defaulting to drafts, and show revision history as expandable Noora summary panels.
• Render comments as permalinkable GitHub-style threads with author avatars, preferring GitHub avatar URLs for GitHub-authenticated users and falling back to Gravatar for email-backed users.
• Expose MCP tools to list, fetch, create, update, and comment on specs, including stale-revision detection and revision payloads.
• Wire Specs into dashboard navigation, forage item actions, route CSS, OpenGraph image metadata, and local seed data for visualization.

Testing

• mix format
• mix compile --warnings-as-errors
• mix test
• curl -I http://localhost:4911/open-graph/specs/1hV9xzlW_QjCu2qSnlx1npZ6B0wVx4H9stWFkx42dUg
• mix ecto.migrate && mix run priv/repo/seeds.exs

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