[maxy-shpfy]

Description

Adds shared folders: server-managed "project" directories that live outside any single
session so multiple sessions can collaborate in one live workspace.

• Bytes live under a new SHARED_ROOT/<id> directory; metadata is tracked in a new
  shared_folders SQLite table (migration 0024) with a SharedFolderStore
  (SQLite + in-memory implementations).
• New REST surface under /api/shared-folders:
  • GET /api/shared-folders — list every shared folder (oldest first).
  • POST /api/shared-folders — create a folder and its backing directory; the creator is
    resolved from their identity when available, and the name defaults to Shared folder N.
• Sessions can mount a shared folder via a new mount host-resource kind. A mount creates a
  one-way symlink at the session root pointing at the shared folder, so the agent reads and
  writes it with its ordinary file tools. mountName is an optional single-segment symlink
  name (defaults to a sanitized form of the folder's name).
• Mounting is one-way — removing the catalog entry leaves the symlink in place; there is
  no unmount.
• Session resource wiring now threads SharedFolderStore through the resource routes and
  seed deps.

Contracts (@tangent/shared), the embed types, and server docs/README are updated to
describe the feature and the new SHARED_ROOT config.

Screenshots

N/A

How to test

1. pnpm --filter @tangent/server db:migrate to apply migration 0024.
2. pnpm dev (local run points SHARED_ROOT at a gitignored .shared folder).
3. POST /api/shared-folders with an optional { "name": "..." } and confirm a 201 with the
   created folder plus a backing directory under SHARED_ROOT/<id>.
4. GET /api/shared-folders and confirm it lists the folder.
5. Add a mount host resource to a session referencing the folderId; confirm a symlink
   appears at the session root and that two sessions mounting the same folder see each other's
   writes live.
6. Run the server suite:
   • pnpm --filter @tangent/server exec tsx --test src/store/sqliteSharedFolderStore.test.ts
   • pnpm --filter @tangent/server exec tsx --test src/routes/sessions/resources.test.ts

Notes

• Adds SHARED_ROOT env var — set it in deployed environments; pnpm dev defaults to
  .shared.
• Mounting is intentionally one-way for this iteration (no unmount).

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