Add agent guideline: documentation belongs in the docs repo (#11492)

Mirrored in AGENTS.md, .github/copilot-instructions.md and CLAUDE.md, with a
matching CodeRabbit path instruction for **/*.md.

.github/copilot-instructions.md

@@ -338,6 +338,7 @@ firmware/
 - Use `assert()` for invariants that should never fail
 - C++17 features are available (`std::optional`, structured bindings, `if constexpr`, etc.)
 - **Keep code comments minimal - one or two lines, max.** Comment only when the _why_ isn't obvious from the code; never restate what the next line does. No multi-paragraph block comments explaining straightforward changes. The diff and commit message carry the rationale; the code carries the behavior.
+- **Documentation does not live in this repo. Do not add it here.** This repository holds firmware code. There is no `docs/` directory - the design documents that used to sit there were published to [meshtastic/meshtastic](https://github.com/meshtastic/meshtastic) in #11488 and the directory was deleted - and it must not come back. Do not create a `.md` file to describe a feature, a configuration surface, an API, a wire format, or a design; write it in the docs repo and link that PR instead. Never leave a write-up behind in the tree: no investigation notes, no mitigation plans, no migration checklists, no "how we got here" narrative, no summaries of what a change did. That is what the PR description and the commit message are for, and they are the only place it belongs. When you do write documentation upstream, write a technical manual, not a novel - what the feature does, the settings it exposes in the user's terms, and the exact API or protocol a client speaks. No story of the debugging journey, no rationale essays, no changelog prose. Concise and factual, as short as the facts allow.
 - **Never compare against `millis()` directly. Use `Throttle`.** `src/mesh/Throttle.h` is the sanctioned way to ask about time, and CI enforces this (`millis-deadline-check` in `.github/workflows/test_native.yml` fails the PR on a new `millis() >` / `< millis()` comparison).
   - `Throttle::isWithinTimespanMs(lastMs, intervalMs)` - true while still inside the cooldown.
   - `Throttle::hasElapsed(lastMs, intervalMs)` - its complement, true once the interval has passed (inclusive `>=`). Prefer this to spelling `!isWithinTimespanMs(...)`.