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/plugin.yaml

@@ -0,0 +1,7 @@
+name: web-exa
+version: 1.0.0
+description: "Exa web search and content extraction. Requires EXA_API_KEY — sign up at https://exa.ai."
+author: NousResearch
+kind: backend
+provides_web_providers:
+  - exa