fix: add PLATFORM_HINTS entry for api_server platform

The API server is a documented, first-class messaging platform with its own
gateway adapter, docs pages, and toolset. But it's the only messaging
platform missing from PLATFORM_HINTS in agent/prompt_builder.py.

Without a platform hint, the agent has no context about the API server's
rendering environment and defaults to markdown-heavy document-style outputs
(code fences, bold, bullet points) — which break on the plain-text frontends
most API server consumers wrap (Open WebUI, custom agents, third-party
bridges).

Adds a generic api_server entry that describes the medium (unknown rendering,
assume plain text) without encoding any specific use case. Individual consumers
can layer additional style guidance via ephemeral system prompts.

Before (DeepSeek V4 Pro via API server, no hint):
  **Sendblue bridge** at /opt/sendblue-bridge - **68MB** on disk

After (same prompt, with hint):
  Sendblue bridge at /opt/sendblue-bridge, 68MB on disk

No breaking changes — new dict entry only. Existing API server consumers see
no behavioral change except for models that previously defaulted to markdown
formatting, which now produce cleaner plain-text output.

agent/prompt_builder.py

@@ -513,6 +513,12 @@ PLATFORM_HINTS = {
         "image and is the WRONG path. Bare Unicode emoji in text is also not a substitute "
         "— when a sticker is the right response, use yb_send_sticker."
     ),
+    "api_server": (
+        "You're responding through an API server. The rendering layer is unknown — "
+        "assume plain text. No markdown formatting (no asterisks, bullets, headers, "
+        "code fences). Treat this like a conversation, not a document. Keep responses "
+        "brief and natural."
+    ),
 }
 
 # ---------------------------------------------------------------------------

tests/agent/test_prompt_builder.py

@@ -788,6 +788,7 @@ class TestPromptBuilderConstants:
         assert "discord" in PLATFORM_HINTS
         assert "cron" in PLATFORM_HINTS
         assert "cli" in PLATFORM_HINTS
+        assert "api_server" in PLATFORM_HINTS
 
     def test_cli_hint_does_not_suggest_media_tags(self):
         # Regression: MEDIA:/path tags are intercepted only by messaging