fix(plane): sandbox-only fail-closed guard for Plane writes from test process (ORCH-117)

Close the root class of incident ORCH-114: a pytest/worktree process performed a
REAL write (PATCH issues state=<Done> + comment) against the PRODUCTION Plane
project, because test/staging processes inherit the live Plane token
(PLANE_HEADERS/PROJECT_ID are captured at import — a post-hoc env/token swap is a
no-op) and nothing forced them to write only to the sandbox. Symmetric to the
existing _no_telegram autouse floor.

- New pure never-raise leaf src/plane_write_guard.py (decide/audit_block/
  audit_allow), wired into the 3 plane_sync write primitives (update_issue_state /
  add_comment / _set_issue_state_direct) via _guard_allows_write, AT CALL TIME,
  before any network step. Active ONLY in a test process (pytest in sys.modules /
  PYTEST_CURRENT_TEST); live + staging runtimes (uvicorn) are a strict no-op.
- In a test process: default-deny. A write is allowed iff opt-in
  (plane_test_write_enabled) AND target project in the sandbox allowlist
  (plane_test_sandbox_projects, default = the one SANDBOX id). Prod is blocked even
  with opt-in (allowlist sandbox-only); unresolved project -> block (fail-closed).
- Independent second layer: tests/conftest.py::_plane_sandbox_only autouse floor.
  Intentionally NO prod-block kill-switch (anti back-door, NFR-6).
- Audit: block -> loud ERROR; sandbox-allow -> INFO.
- Bypass fixtures for the 3 (+1) pre-existing tests that assert on the mocked
  write primitive's httpx call (header/URL/state logic), the guard is no Quality
  Gate: STAGE_TRANSITIONS / QG_CHECKS / check_* / machine-verdict / DB schema
  untouched.
- Tests: tests/test_orch117_plane_write_isolation.py (TC-01 mandatory ORCH-114
  regression + TC-02..TC-14). Docs: CLAUDE.md, architecture/README.md,
  operations/INFRA.md, .env.example, CHANGELOG.md.

Refs: ORCH-117
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -24,6 +24,19 @@ ORCH_PLANE_BOT_REVIEWER=
 ORCH_PLANE_BOT_TESTER=
 ORCH_PLANE_BOT_DEPLOYER=
 ORCH_PLANE_BOT_STREAM=
+# ORCH-117: sandbox-only fail-closed guard for Plane WRITES from a test/worktree
+# process (regression of ORCH-114, where pytest mutated a live prod board issue).
+# In the live runtime (uvicorn, no pytest) the guard is a no-op; in a test process
+# it BLOCKS every Plane write unless BOTH the opt-in is true AND the target project
+# is in the sandbox allowlist. Defaults are SAFE (default-deny): leave both as-is.
+#   ORCH_PLANE_TEST_WRITE_ENABLED -> opt-in for REAL Plane writes from a test process.
+#     false (default) = no test may write to Plane. NOT a kill-switch for the prod
+#     block: even true, only the sandbox allowlist below is writable (a prod write
+#     from pytest stays impossible).
+#   ORCH_PLANE_TEST_SANDBOX_PROJECTS -> CSV allowlist of sandbox project ids the
+#     opt-in may write to. Default = the single SANDBOX project; empty = none.
+ORCH_PLANE_TEST_WRITE_ENABLED=false
+ORCH_PLANE_TEST_SANDBOX_PROJECTS=8c5a3025-4f9d-4190-b79f-fa06276bb27e
 # Telegram live-tracker / alerts (empty -> notifications are logged, not sent).
 ORCH_TELEGRAM_BOT_TOKEN=
 ORCH_TELEGRAM_CHAT_ID=