docs: refresh stale platform/LOC/test counts; clarify gateway vs plugin platforms

AGENTS.md is the AI-assistant entry doc, so its counts get used as ground
truth. Several values had drifted, and the same drift had spread to a few
user-facing surfaces. Fixing all of them in one commit so the count claims
agree and clearly distinguish gateway-core from plugin-shipped platforms.

AGENTS.md:
- run_agent.py "~12k LOC" → "~14k LOC as of 2026-05-03" (actual 14,097)
- cli.py     "~11k LOC" → "~12k LOC as of 2026-05-03" (actual 12,043)
- tools/environments/ list now lists all 7 user-selectable terminal backends
  in canonical order, matching tools/terminal_tool.py:2214-2215
- gateway/platforms/ list adds yuanbao and wecom_callback; the 19 names
  match the user-facing list at website/docs/integrations/index.md
- plugins/ tree now mentions plugins/platforms/ (irc, teams)
- tests/ snapshot "~15k tests across ~700 files as of Apr 2026" →
  "~19k tests across ~890 files as of 2026-05-03"

User-facing count claims:
- hermes_cli/tips.py:195 — "19 platforms" → "21 messaging platforms" with
  IRC and Microsoft Teams added to the named list
- website/docs/index.md:49 — "6 terminal backends" → "7 terminal backends:
  ..., Vercel Sandbox" (also corrected by PR #19044; same edit content)
- website/docs/index.md:50 — "15+ platforms from one gateway" → "21+ messaging
  platforms (19 in the gateway, plus IRC and Microsoft Teams via plugins)"
- website/docs/integrations/index.md:83-85 — "15+ messaging platforms" → "19+",
  added yuanbao to the linked list. The surrounding text scopes it to "configured
  through the same gateway subsystem", so plugin platforms (IRC, Teams) are
  intentionally not in this list
- website/scripts/generate-llms-txt.py:205 — "15+ platforms" → "21+ messaging
  platforms — 19 native to the gateway plus IRC and Microsoft Teams via plugins"

LOC and date stamps follow the existing AGENTS.md "as of <date>" convention
(line 56 already used this pattern). Source of truth for the gateway count is
gateway/config.py:130-148 (PlatformID enum); plugin platforms live in
plugins/platforms/.

Out of scope:
- RELEASE_v0.9.0.md historical "16 platforms" claim (immutable history)
- userStories.json verbatim user quotes
- Programmatic count generation from gateway/config.py + plugin manifests
  is a worthwhile build-system change but separate from these content fixes

website/scripts/generate-llms-txt.py

@@ -202,7 +202,8 @@ def emit_llms_index() -> str:
     lines.append(
         "> The self-improving AI agent built by Nous Research. A terminal-native "
         "autonomous coding and task agent with persistent memory, agent-created skills, "
-        "and a messaging gateway that lives on 15+ platforms (Telegram, Discord, Slack, "
+        "and a messaging gateway that lives on 21+ messaging platforms — 19 native to "
+        "the gateway plus IRC and Microsoft Teams via plugins (Telegram, Discord, Slack, "
         "SMS, Matrix, ...). Runs on local, Docker, SSH, Daytona, Modal, or Singularity "
         "backends. Works with Nous Portal, OpenRouter, OpenAI, Anthropic, Google, or any "
         "OpenAI-compatible endpoint."