Replace workspaces with nested collections

[mshddev]

Summary

Workspaces are gone. Documents are top-level with a globally unique slug, and collections are optional folders that nest like a file tree. A document lives at the root or in exactly one collection.

Breaking. This changes every document URL, the API surface, the database schema, and the skill's script flags.

What changed

• URLs. Browser: /docs/<slug>, /collections/<slug>, /collections, /archive?collection=<slug>. API: /api/docs/... and /api/collections/.... Old /workspaces paths return 404.
• Collections. GET /api/collections returns a flat list with parentSlug, documentCount, and childCount; the client builds the tree. POST creates (optionally inside a parent), PATCH renames or reparents, DELETE needs an empty collection. Reparenting into your own descendant is rejected.
• Publishing. The publish body takes an optional collectionSlug (null for the root, omitted to leave an existing document where it is). Republishing identical content with a new collection moves it without a new version but rotates the ETag.
• Migration to schema 10. Documents from the default workspace move to the root, every other workspace becomes a root collection, and the migration refuses to run if a document slug exists in more than one workspace.
• Web. The dashboard is a folder view with breadcrumbs. Browsing shows a folder's direct contents; searching reaches into subfolders and labels each match with its path. The collections page manages the tree.
• Skill. Both scripts drop --workspace; publish-document.mjs gains --collection <slug> and --root. The watcher event no longer carries workspaceSlug. Reinstall the skill after upgrading.

Verification

• Typecheck, build, and tests are green: 102 server tests, 69 web tests.
• The migration ran against a copy of a real schema-9 database (45 documents, 4 workspaces): foreign keys clean, all documents, versions, and feedback batches present.
• End-to-end pass on an isolated stack: both skill scripts (create, feedback watcher, republish with --feedback-match, move to root, stale ETag 412), and the browser flows for dashboard, folder view, move, archive and unarchive, a three-level collection tree with reparenting and the delete guard, root search into nested folders, and submitting a comment plus instruction from the review page.

Not in this PR

The architecture docs under docs/ still describe workspaces as design history. No version bump; the changelog has an Unreleased entry.

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