feat(web): exa plugin — first multi-capability migration (search + extract)

Migrates Exa from the inline `_exa_search()` / `_exa_extract()` helpers in
tools/web_tools.py to a bundled plugin at plugins/web/exa/.

This is the first plugin in this PR to advertise supports_extract=True,
exercising the multi-capability ABC path that the initial three migrations
(brave_free, ddgs, searxng — all search-only) did not cover.

Both Exa methods are sync — the SDK is sync-only. The web_extract_tool
dispatcher in tools/web_tools.py will continue to call them inline until
Task "dispatch-extract-all" cuts it over to the registry.

Behaviour preserved bit-for-bit aside from the ABC method-name change:
  - is_configured()  -> is_available()
  - provider_name()  -> name (property)
  - "exa" stays as the registered name
  - Module-level `_exa_client` cache + lazy `from exa_py import Exa`
    preserved at the new location.
  - Errors (ValueError for missing API key, ImportError for missing SDK,
    generic Exception) caught and surfaced as {"success": False, "error": ...}
    instead of raising.

Adds "exa" to _WEB_PLUGIN_SKIPLIST in hermes_cli/tools_config.py so the
hardcoded TOOL_CATEGORIES["web"] row and the plugin-injected row don't
duplicate during the spike. The skip-list goes away in the cleanup phase
along with the hardcoded row.

The legacy inline `_exa_search` / `_exa_extract` / `_get_exa_client` /
`_exa_client` in tools/web_tools.py are NOT deleted yet — the dispatcher
still references them. They go away in the next dispatcher-cutover commit.

E2E verified:
  - Plugin discovers + registers
  - .supports_search/.supports_extract/.supports_crawl = (True, True, False)
  - .get_setup_schema() returns the picker row shape
  - resolve(): explicit exa + EXA_API_KEY -> exa; without key -> exa (registered
    but unavailable, dispatcher surfaces "EXA_API_KEY not set" error)

plugins/web/exa/__init__.py

@@ -0,0 +1,15 @@
+"""Exa web search + extract plugin — bundled, auto-loaded.
+
+Backed by the official Exa SDK (``exa-py``). Both search and extract are
+sync; the dispatcher in :mod:`tools.web_tools` handles the wrap when the
+caller is async.
+"""
+
+from __future__ import annotations
+
+from plugins.web.exa.provider import ExaWebSearchProvider
+
+
+def register(ctx) -> None:
+    """Register the Exa provider with the plugin context."""
+    ctx.register_web_search_provider(ExaWebSearchProvider())