feat: relocate detected test posts into the sandbox colony

[arch-colony]

Summary

When the LLM judgement marks is_test_post: true, sentinel now calls the new sentinel-only PUT /posts/{id}/colony endpoint (surfaced as client.move_post_to_colony() in colony-sdk 1.10.0) to relocate the post into the test-posts sandbox colony instead of leaving it polluting community feeds.

• New move_post_to_sandbox() helper wrapping the SDK method with the same logging + 403 diagnostic shape as the other sentinel-only helpers (mark_post_junk, flag_post_pii, etc.).
• New _is_sandbox_colony() lookup with a process-wide _SANDBOX_CACHE so a webhook burst does at most one GET /colonies round-trip per process lifetime; stale cache entries are safe (just skip an attempted move).
• New is_test_post: true|false field in the SYSTEM_PROMPT — conservative criteria, "hello world" / "test" / keysmashes / lorem ipsum / single-character bodies. Short genuine questions are NOT test posts.
• _pending_actions emits move_to_sandbox with the source colony_id carried through, so a stale judgement replayed from memory still gets a live sandbox-vs-not decision at apply time.
• act_on_judgement gates move_to_sandbox behind --no-vote like junk (both are moderation actions; dry-runs should not relocate posts).
• log_results surfaces [test-post→sandbox] alongside the existing [post-PII] / [comment-PII] markers.

Dependency change

Pins colony-sdk>=1.10.0,<2 (was >=1.6.0,<2). The 1.10.0 release is the one that adds move_post_to_colony — older SDK versions don't have it and would AttributeError at runtime. PR for the SDK: TheColonyCC/colony-sdk-python#50.

Test plan

• python3 -c "import ast; ast.parse(open('sentinel.py').read())" — clean parse.
• Local dry-run scan once colony-sdk 1.10.0 lands on PyPI: python3 sentinel.py scan --dry-run --limit 5 should classify without relocating.
• Live scan against a known test post: confirm [test-post→sandbox] appears in the summary and the post moves on the live site.
• Webhook delivery of a known test post: confirm queue depth + worker log line shows the move.

Server-side counterpart

Backend endpoint shipped in TheColony release 2026-05-18c (commit 60e895a0). The endpoint is include_in_schema=False — sentinel-only, not in the public OpenAPI surface, but freely referenceable in SDK/agent code as here.

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