Connectors multi-instance, OpenSandbox storage backend, per-agent plugin toggle

[jubaoliang]

Summary

This PR bundles several unrelated feature/chore changes that were sitting together in the working tree, split into one commit per requirement for reviewability.

• feat(connectors): support multiple connector instances of the same kind, and shared connectors visible across users (list_visible), with DB migration 013_connector_instances (schema v13).
• feat(storage): add OpenSandbox as a remote sandbox storage backend kind (probe/health-check, execution support, previewability rules).
• feat(plugins): add a per-agent plugin enable/disable toggle; move plugin configuration UI from the global Plugins admin page into the agent's Personalization tab.
• feat(experts): add catalog display metadata (emoji/label/summary) to ~30 bundled expert skills' SKILL.md frontmatter — no logic changes.
• chore: misc unrelated fixes — README desktop client download table, RemoteBrowser empty-state mascot icon, vite dev proxy xfwd: true fix (preserves browser origin for OAuth callbacks).

Known deviations (shared/interwoven files)

A few files touch more than one of the above features and were kept whole in a single commit rather than split line-by-line:

• src/octop/infra/errors.py, backend i18n/{en,zh}.json, and dashboard locales/{en,zh}.json are committed in full with the connectors commit (commit 1), since ErrorCode.CONNECTOR_NAME_TAKEN is referenced by the connectors router at that point. This means commit 1 also carries some strings/enum members that only become "live" once the storage/plugins commits land (safe — unused additions, nothing missing).
• src/octop/infra/agents/manager.py and tests/unit/agents/test_mcp_tool_cache.py are committed whole with the plugins commit (commit 3), even though manager.py also contains connectors-related (list_visible) and opensandbox-related changes, because the new integration test test_agent_plugin_switch_defaults_on_and_preserves_tools depends on the per-agent plugin logic in that file to pass.

Verification

make all   # format-all + lint + typecheck + test

Result: 2949 passed, 8 skipped — all green (format/lint/typecheck run before tests in the all target; test phase completing confirms all prior phases passed).

Commits

1. feat(connectors): support multiple connector instances and shared connectors
2. feat(storage): add OpenSandbox remote sandbox storage backend
3. feat(plugins): add per-agent plugin toggle, move plugin config to Personalization
4. feat(experts): add catalog display metadata to bundled skills
5. chore: misc unrelated fixes