docs(contributing): add CONTRIBUTING-WITH-AI.md companion guide

Companion to CONTRIBUTING.md targeting external PRs whose diff was
substantially authored by an AI coding assistant. Adds five gates the
existing doc does not cover:

  - Research before code (issue still valid, decentralisation fit)
  - Build and install both flavours (Play + F-Droid)
  - Performance and resource hygiene (main-thread, coroutines,
    LocalCache reuse, relay subscriptions, KMP source sets, lambda Log)
  - Regression test plan alongside feature test plan
  - Don't-touch list (signer/KeyStore, release pipeline, NIP direction)

Filename deliberately avoids AGENTS.md to keep internal dev-team
agents unaffected by auto-load conventions. CONTRIBUTING.md and the
PR template each gain a one-line pointer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -57,6 +57,10 @@ appreciated when an assistant did the bulk of the diff. -->
 - [ ] Drafted with AI assistance, manually reviewed and tested
 - [ ] Written by hand
 
+If "Drafted with AI assistance" is ticked, also read
+[`CONTRIBUTING-WITH-AI.md`](CONTRIBUTING-WITH-AI.md) for the additional
+gates that apply to AI-authored PRs.
+
 ## License
 
 - [ ] By submitting this PR, I agree to license my contribution under the