docs(prompt): document platform_hints config override

Adds a 'Customizing platform hints' section to the Prompt Assembly
developer guide covering the append/replace/shorthand shapes, the
defensive fallback, and the cache-stable lifecycle (stable tier,
resolved at build time).

website/docs/developer-guide/prompt-assembly.md

@@ -116,6 +116,43 @@ You are a CLI AI Agent. Try not to use markdown but simple text
 renderable inside a terminal.
 ```
 
+## Customizing platform hints
+
+The platform hint (Layer 10 above) is the per-surface guidance Hermes
+injects for Telegram, WhatsApp, Slack, CLI, and other platforms — for
+example "you are on a terminal, avoid Markdown." The built-in defaults
+live in `PLATFORM_HINTS` (`agent/system_prompt.py`); plugin-provided
+platforms supply theirs through the platform registry.
+
+An administrator can append to or replace a single platform's hint from
+`config.yaml` via the top-level `platform_hints` key, without touching
+any other platform:
+
+```yaml
+platform_hints:
+  whatsapp:
+    append: >
+      When tabular output would be useful, invoke the table_formatting
+      skill instead of emitting a Markdown table.
+  slack:
+    replace: "You are on Slack. Keep responses tight and avoid wide tables."
+  telegram: "Prefer short messages; split long answers."   # shorthand = append
+```
+
+- `append` — keep the built-in hint and add the extra text after it.
+- `replace` — substitute the built-in hint entirely.
+- A bare string — shorthand for `append`.
+- `replace` wins over `append` when both are present.
+- A malformed entry is ignored defensively and falls back to the
+  unmodified default, so a bad config value can never break prompt
+  assembly or leak across platforms.
+
+The override is resolved when the system prompt is built (session start,
+and again on compaction since that rebuilds the prompt). It produces a
+byte-stable hint for a fixed config, so it lives in the **stable** tier
+alongside the built-in hint and does not break prompt caching — it is
+not a live mid-session mutation of a frozen prompt.
+
 ## How SOUL.md appears in the prompt
 
 `SOUL.md` lives at `~/.hermes/SOUL.md` and serves as the agent's identity — the very first section of the system prompt. The loading logic in `prompt_builder.py` works as follows: