refactor(tools): drop hardcoded web picker rows + skiplist; plugins are sole source

Removes the seven hardcoded TOOL_CATEGORIES["web"] provider rows that
duplicated the plugin-registered providers, and deletes the
_WEB_PLUGIN_SKIPLIST that existed to prevent duplicate picker rows
during the migration. The Web Search & Extract category now derives its
provider rows entirely from agent.web_search_registry via
_plugin_web_search_providers(), matching how Spotify, Google Meet, and
the image_gen plugins are surfaced.

Removed (deduplicated against plugin schemas):
  - Firecrawl Cloud         → plugins.web.firecrawl
  - Exa                     → plugins.web.exa
  - Parallel                → plugins.web.parallel
  - Tavily                  → plugins.web.tavily
  - SearXNG                 → plugins.web.searxng
  - Brave Search (Free Tier) → plugins.web.brave_free
  - DuckDuckGo (ddgs)       → plugins.web.ddgs (post_setup hook preserved)

Retained in TOOL_CATEGORIES["web"]:
  - Nous Subscription   — requires requires_nous_auth +
                          managed_nous_feature + override_env_vars
                          to drive the managed-gateway UX. Not a
                          provider — a different *setup flow* for the
                          firecrawl backend.
  - Firecrawl Self-Hosted — points firecrawl at a private Docker URL
                            via FIRECRAWL_API_URL only. Same reason:
                            UX setup-flow row, not a provider.

These two rows describe alternative auth/billing paths for the
firecrawl backend; they intentionally share web_backend="firecrawl"
with the plugin row but light up different env-var prompts.

Plugin schema extensions
------------------------
- ddgs plugin's get_setup_schema() now emits `post_setup: "ddgs"` so
  selection still triggers the pip-install hook in _run_post_setup().
- _plugin_web_search_providers() passes `post_setup` through verbatim
  when present in the schema (other future plugins like camofox / a
  hypothetical playwright-web plugin can opt in the same way).
- Picker rows now carry both `web_backend` (legacy field consumed by
  setup + selection helpers) and `web_search_plugin_name`
  (informational marker), so behavior is identical between hardcoded
  and plugin-registered rows.

Net diff
--------
- hermes_cli/tools_config.py: -141/+50 lines (~91 lines net)
- plugins/web/ddgs/provider.py: +7/-4 (post_setup field + badge polish)

Verified
--------
- Compile-clean for both files
- Picker shows: 2 hardcoded rows (Nous Subscription, Firecrawl
  Self-Hosted) + 7 plugin rows (alphabetically: Brave Search,
  DuckDuckGo, Exa, Firecrawl, Parallel, SearXNG, Tavily). DuckDuckGo
  row carries post_setup="ddgs" for first-time install.
- 173 web-specific tests still pass.

plugins/web/ddgs/provider.py

@@ -95,7 +95,10 @@ class DDGSWebSearchProvider(WebSearchProvider):
     def get_setup_schema(self) -> Dict[str, Any]:
         return {
             "name": "DuckDuckGo (ddgs)",
-            "badge": "free",
-            "tag": "No API key — community ddgs package (pip install ddgs).",
+            "badge": "free · no key · search only",
+            "tag": "Search via the ddgs Python package — no API key (pair with any extract provider)",
             "env_vars": [],
+            # Trigger `_run_post_setup("ddgs")` after the user picks this row
+            # so the ddgs Python package gets pip-installed on first selection.
+            "post_setup": "ddgs",
         }