feat(workspaces): block creating a workspace whose slug is archived

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+# Plan — Warn when creating a workspace whose slug belongs to an archived workspace
+
+Status: requested by user, ready to implement
+Branch: `feature/archived-workspace-create-warning`
+Scope: **frontend only**. No backend change (the backend already exposes everything needed).
+
+---
+
+## 1. User requirement
+
+> "I want the frontend to warn me if I try to create a workspace that already
+> exists and is archived (it should say you can't)."
+
+So: when the user submits the **New workspace** form with a slug that matches an
+existing **archived** workspace, the frontend must **not** create/ensure it and
+must show a message saying it **can't** be created because a workspace with that
+name already exists and is archived.
+
+## 2. Current behavior (why it's wrong today)
+
+- `HomeHeader.svelte` (`src/features/workspaces/ui/HomeHeader.svelte`) — the
+  New-workspace popover. `submit()` validates the slug with `isValidSlug`, then
+  calls `onCreate(slug)` and shows `result.error` in the existing `slugError`
+  slot when `result.ok === false`.
+- `WorkspacesHome.svelte` → `createWorkspace(slug)` calls `store.ensure(slug)`,
+  then `store.refresh()`, `tiles.placeFirstEmpty(slug, …)`, and enters rename
+  mode (`renamingId = slug`).
+- `store.ensure` → `http.ensure` → backend `PUT /workspaces/:id`, which is
+  **idempotent**: for an existing archived workspace it just returns it
+  (`archived: true`) — no error. The active list (`GET /workspaces`, archived
+  excluded by default) doesn't contain it, yet the home grid places a tile for
+  it and enters rename mode. Silent and confusing.
+
+Backend facts (no change needed):
+- `GET /workspaces/:id` → the workspace incl. `archived: boolean`, or 404.
+  Frontend adapter already has `http.get(id)` (`src/features/workspaces/adapter/http.ts`)
+  — note it currently returns `null` for **both** 404 and network/other errors.
+- `GET /workspaces?archived=true` → archived-only list (the store's
+  `refreshArchived()` / `archivedList`, used by the Archived view).
+
+## 3. Required behavior
+
+1. On submit, **before** calling `ensure`, determine whether a workspace with
+   that slug exists and is archived. Use an authoritative check against the
+   backend (e.g. `GET /workspaces/:id`) — do **not** rely on `archivedList`
+   alone, since it's only loaded when the Archived view is opened.
+2. If it exists and is archived:
+   - do **not** call `ensure`, do not refresh/place a tile, do not enter rename;
+   - keep the popover open and show the error in the existing `slugError` slot,
+     clearly saying it can't be created because an archived workspace with that
+     name already exists. Suggested wording:
+     `A workspace named "<slug>" already exists and is archived — you can't create it.`
+     (It's fine to append a short hint that it can be restored from Archived
+     workspaces, but don't add new UI controls for that.)
+3. If it doesn't exist, or exists and is **not** archived → today's behavior,
+   unchanged.
+4. If the existence check itself fails (network/server error), don't silently
+   swallow it in a way that creates an archived workspace's tile: either surface
+   an error, or proceed exactly as today — pick one, keep it simple, and justify
+   it in the report. (Distinguishing 404 from "request failed" may require a
+   small adapter/store change; that's in scope.)
+5. While the check is in flight, the existing `submitting` state should cover it
+   (no double submits).
+
+Follow the frontend's architecture rules in `AGENTS.md` (where decision logic
+lives, store/adapter boundaries, Svelte 5 runes conventions, styling).
+
+## 4. Out of scope
+- Creating a slug that matches an existing **active** workspace (unchanged).
+- Any backend change (e.g. making `PUT /workspaces/:id` reject archived ids).
+- New UI for unarchiving from the popover.
+
+## 5. Tests
+- Unit/store test: creating an archived slug returns `ok: false` with the
+  "can't create" message and does **not** call `ensure`.
+- Non-archived existing slug and brand-new slug → `ensure` is called as before.
+- Check-failure path per your choice in 3.4.
+- Component test (`HomeHeader.test.ts` or `WorkspacesHome` level, whichever
+  matches existing patterns): the message renders in the popover and the
+  popover stays open.
+
+## 6. Verification gate
+`bun install`, `bun run typecheck`, `bun run test`, `bun run check`,
+`bun run build` — all green. Commit to
+`feature/archived-workspace-create-warning` only. Do not merge or push.