Projects: shareable, registry-backed, container-mounted; drop ticket board

Rework projects from single-user leftovers into shareable endeavours.

- DB: move `projects` from the owner bucket to the registry (system.db,
  not encrypted); add `owner_user_id` + `slug` (drop free `path`); new
  `project_members(project_id, user_id, can_write)` mirroring
  `shared_folder_members`. Drop `project_tickets` entirely. Only user↔agent
  conversations stay encrypted (per-user DB) — each member keeps a private
  project chat. Registry home dissolves the cross-DB-FK problem.
- Filesystem/container: on disk `{WD}/projects/{owner_userid}/{slug}`,
  agent/container path `projects/{owner_username}/{slug}`. Two-segment routing
  in UserFs (ProjectMount + host_base_and_tail arm) and a second loop in
  build_user_fs; read-only members get a :ro mount. Reuse the shared-folder
  remount machinery (refresh_user_shared_folders -> refresh_user_mounts).
- Remove the ticket system: ProjectTicketManager, UserContext.tickets, its
  wiring, and the project_tickets references in scheduled_jobs/cron.
- API: repoint handlers to the registry pool + membership scoping. Sharing is
  self-service — owner or any write-member may add/remove members and set
  read/write; only the owner deletes; the owner cannot be removed. New
  POST/DELETE /api/projects/{id}/members[/{user_id}]. Seed `@fs_any allow
  projects/*`; build_runtime_run_context sets working_directory to the agent
  path and drops the host-path allow_fs_writes.
- Frontend: create form without the free path field, owner/read-write badges;
  the detail page becomes header + description + sharing panel + Open chat + a
  file-explorer placeholder (the future primary surface). i18n en/it/fr.

agents/project-coordinator/AGENT.md

@@ -28,7 +28,7 @@ Delegate work to these task specialists via `execute_task` / `execute_subtask`:
 
 Your system prompt already contains, without you asking:
 
-- The project's **name**, **description**, and **working directory** (the project root — all relative file paths resolve there). You have **pre-authorized write access** to the project tree, so writing files there needs no approval.
+- The project's **name**, **description**, and **working directory** (the project root — all relative file paths resolve there). You have **pre-authorized write access** to the project tree, so writing files there needs no approval. A project may be **shared** with other members (read-only or read-write): anything you write into the project folder is visible to everyone it is shared with, so keep private, user-specific notes in `user-memory/` rather than in a shared project.
 - **`user-memory/index.md`** and **`shared-memory/index.md`** — the indexes of your **private** memories (who the user is, their preferences, people, other projects) and the group's **shared** memories. Both are injected automatically. Before acting on anything personal, read the specific note the index points to — don't rely on the one-line summary alone.
 - **`SKALD.md`** at the project root — this project's **living diary** (see below). It is injected automatically; if it doesn't exist yet you'll see a `(file not created yet)` placeholder.