fix(backend): support mkdir/delete/move on OpenSandbox agent workspaces

[JasonBuildAI]

Summary

Any expert on an opensandbox storage backend currently cannot start: octop_harness's OpenSandbox supports execute / upload_files / download_files, but unlike DockerSandbox it never defines mkdir_path / delete_path / move_path, and BackendWorkspace requires those three primitives on any backend without a host mount. The first workspace mutation therefore raises BackendOperationNotSupportedError: mkdir is not supported by backend type 'OpenSandbox' and the expert dies during startup. This PR adds the three primitives in Octop's backend protocol-compat layer, so OpenSandbox-backed experts start and workspace seeding / session files work like on the Docker backend.

Fixes #1813.

Target branch

• Base is develop (feature / fix — default)
• Base is main (release/* or hotfix/* only)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore
• Release / hotfix

Test plan

Reproduced first, then fixed (octop @ this branch, octop_harness 1.0.2, Python 3.12.12, Windows 10 zh-CN).

1. Minimal repro — real OpenSandbox class + fake remote SDK, no live sandbox. Before the change:

backend class: OpenSandbox
has mkdir_path: False

REPRO: workspace init FAILED

Traceback (most recent call last):
  File "...\_repro1813.py", line 60, in main
    workspace.mkdir("skills/demo")
  File "...\octop_harness\backends\workspace.py", line 1024, in mkdir
    raise BackendOperationNotSupportedError("mkdir", type(self._backend).__name__)
octop_harness.backends.utils.BackendOperationNotSupportedError: mkdir is not supported by backend type 'OpenSandbox'

The same repro after the change:

backend class: OpenSandbox
has mkdir_path: False

OK: sandbox commands = ['mkdir -p /skills/demo', 'mkdir -p /skills/demo && mv /SOUL.md /skills/demo/SOUL.md', 'rm -rf /skills/demo']

2. The new unit tests are sensitive to the fix: uv run pytest tests/unit/backend/test_compat.py -q -k sandbox → 2 failed, 3 passed without it, 21 passed with it.
3. uv run pytest tests/unit/backend -q → 42 passed, 1 skipped.
4. uv run ruff check + uv run ruff format --check on the two touched files → All checks passed! / 2 files already formatted; uv run mypy src/octop → Success: no issues found in 570 source files.
5. uv run pytest -n auto -m "not live" → 27 failed, 4430 passed, 201 skipped. All 27 failures are pre-existing environment issues on this box and unrelated to this change: 19 of them (tests calling Path.read_text() without an explicit encoding under a zh-CN Windows GBK locale, in tests/unit/db and tests/unit/backup) pass when the suite is rerun with PYTHONUTF8=1; the remaining 8 are the desktop vendor-wheels shell-script tests, which invoke C:\Windows\System32\bash.EXE (a WSL stub here, not a usable bash). No failing module imports infra/backend.

Notes:

• This machine has no make; the commands above are the make all equivalents (ruff check / ruff format --check / mypy / pytest) run directly.

• Why fix this in Octop instead of octop-harness: harness 1.0.2 is already released with the primitives missing, and infra/backend/compat.py is the layer Octop already uses to bridge released backends (it adapts the legacy deepagents_backends protocol and the S3 listing gap). The patch is idempotent — it skips backends without sandbox_fs, keeps any existing implementation, and no-ops as soon as the harness backend defines the three methods — so a future harness/deepagents release that adds them automatically supersedes it. A harness-side fix (or lifting the primitives onto deepagents' BaseSandbox, as suggested in the issue) is still the better long-term home; this PR unblocks the released versions.

• make all passes locally (no make on this machine; equivalent commands above)

• Added/updated tests

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (if needed)

AI disclosure: this change was drafted with AI assistance at my request, under my direction and review.