Old pre-decomposition import paths are gone: plugin compat layer removed on schedule (#126164)

* refactor(plugins): remove the Sep 2026 decomposition compat layer on schedule

The PLUGIN-COMPAT layer (2776813df3 + d63e380324 + 0a5164cebe) kept pre-#102117 import paths
alive for external plugins until 2026-09-14. That window closed two weeks ago; since then the loader
has already been skipping plugins that use the old paths. This removes the layer itself:

- 328 appended `PLUGIN-COMPAT` blocks (lazy `__getattr__` pointer tables, re-exported third-party
  names, restored dead definitions) and the three re-export stub modules
  (gateway/startup_watchdog, hermes_cli/observability/relay_runtime, tools/environments/modal_utils)
- COMPAT_MANIFEST.md, compat_manifest.json, scripts/check_compat_pointers.py and its lint step
- the reporting surfaces: CLI banner notice, `hermes plugins compat`, the `hermes doctor` section,
  the post-update notice, the Desktop one-time dialog, the loader's pre-import skip and the
  `plugins.allow_deprecated_imports` escape hatch

An external plugin that still imports an old path now fails to load with its ImportError as the
reason in `hermes plugins list`, the same path as any broken plugin.

hermes_cli/plugin_compat.py stays as three inert stubs (compat_report, removal_in_effect,
summary_lines): an already-running pre-removal `hermes update` lazy-imports them after the checkout
swap (tests/compat/old_updater_surface.json).

In-tree fallout, both already dead: hermes_cli/setup.py::_check_espeak_ng (no callers; its
`shutil` came from a compat block) and gateway/config.py::SessionResetPolicy ("retained solely for
the scheduled plugin-compat window"). Two test_run_agent patches targeted the removed
`run_agent.handle_function_call` pointer; they now patch `model_tools.handle_function_call`, the
seam production reads, like every sibling test in that file.

* chore: retrigger CI (zero-job startup_failure phantom)

* test: drop resolution allowlist rows for the two deleted which() sites

hermes_cli/setup.py::_check_espeak_ng (dead) and tools/skillevaluator_scan.py::scanner_available
(a restored definition inside a PLUGIN-COMPAT block) no longer exist; the stale-row gate requires
their allowlist entries go with them.

tools/code_kernel_remote.py

@@ -431,11 +431,3 @@ def _run_attached_cell(kernel: RemoteKernel, key: Tuple, code: str, *, env, task
     if cell_status == "error" and result["traceback"]:
         result["error"] = result["traceback"].strip().splitlines()[-1]
     return result
-
-
-# ---- BEGIN PLUGIN-COMPAT (revert-scheduled; see COMPAT_MANIFEST.md) ----
-# Names external plugins imported from this module before the Sep 2026 decomposition.
-# Internal code MUST NOT use these (scripts/check_compat_pointers.py fails CI if it does).
-# The whole block is removed by reverting the commit that added it.
-import base64  # noqa: F401,E402
-# ---- END PLUGIN-COMPAT ----