Pluginify provider/platform/terminal backends

Move provider adapters (anthropic, bedrock, azure), platform adapters
(telegram, slack, discord, feishu, dingtalk, matrix), and terminal backends
(modal, daytona) out of core into plugins/ workspace members. Core references
them via the plugin registries (get_provider_namespace / get_provider_service /
get_tool_provider / get_credential_pool_hook) instead of direct imports.

- Provider/platform/terminal adapters relocated under plugins/; pyproject
  extras reference workspace members; nix variants aggregate per-platform extras.
- Anthropic credential discovery + OAuth-masquerade guard live in the plugin's
  credential_pool_hook; browser-open guarded by _can_open_graphical_browser.
- Vercel AI Gateway + Vercel Sandbox removed (upstream deletion); get_bedrock_model_ids
  removed (replaced by bedrock_model_ids_or_none + discover_bedrock_models).
- Terminal backends resolve ModalEnvironment / DaytonaEnvironment lazily from
  the plugin registry.
- uv.lock regenerated against the pluginified workspace.

Plugin test suites updated for the relocation: imports point at
hermes_agent_<plat>.adapter, caplog logger-name filters and monkeypatch targets
use the new module paths, and credential/rollback tests patch
registries.get_provider_service rather than the removed agent.*_adapter modules.

Verified: zero dead imports of relocated modules in core (import smoke test +
rename-map grep); nix develop succeeds; targeted plugin suites green
(bedrock, anthropic-auxiliary, matrix, dingtalk, feishu, credential_pool,
switch_model_rollback). Remaining full-suite failures are pre-existing on the
pre-merge tree (telegram setUpModule __code__) or environmental (voice/media/
PTY/network-dependent), not introduced here.

.github/workflows/lint.yml

@@ -200,3 +200,22 @@ jobs:
 
       - name: Run footgun checker
         run: python scripts/check-windows-footguns.py --all
+
+  plugin-isolation:
+    # Enforce that core code and core tests never import from plugin packages.
+    # Core must interact with plugins exclusively through the registry layer.
+    # See scripts/check_no_plugin_imports_in_core.py for the rule list.
+    name: Plugin isolation (blocking)
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Set up Python
+        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v5
+        with:
+          python-version: "3.11"
+
+      - name: Run plugin isolation checker
+        run: python scripts/check_no_plugin_imports_in_core.py