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.

scripts/run_tests_parallel.py

@@ -30,7 +30,7 @@ Usage:
 
 Environment:
     HERMES_TEST_WORKERS  Override worker count (default: os.cpu_count())
-    HERMES_TEST_PATHS    Override discovery roots (colon-sep, default: 'tests')
+    HERMES_TEST_PATHS    Override discovery roots (colon-sep, default: 'tests:plugins')
 
 Exit code: 0 if every file's pytest exited 0; 1 otherwise.
 """
@@ -50,7 +50,7 @@ from typing import Dict, List, Tuple
 
 
 # Default test discovery roots.
-_DEFAULT_ROOTS = ["tests"]
+_DEFAULT_ROOTS = ["tests", "plugins"]
 
 # Directories to skip during discovery — these suites require real
 # external services (a model gateway, a docker daemon with a prebuilt
@@ -286,6 +286,7 @@ def _run_one_file(
         stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,
         text=True,
+        env={**os.environ, "HERMES_PARALLEL_RUNNER": "1"},
         # POSIX: place the child at the head of its own process group so
         # _kill_tree can SIGKILL the group atomically.
         # Windows: this maps to CREATE_NEW_PROCESS_GROUP in CPython 3.12+;