docs(release): document self-hosted runner and heap sizing

.claude/skills/releasing-healthchat/SKILL.md

@@ -7,8 +7,9 @@ description: Cut a HealthChat Google Play release of element-x-android by taggin
 
 Releases are driven by git tags of the form `healthchat-v<version>` pushed to
 `origin`. The tag triggers `.github/workflows/healthchat-release-playstore.yml`,
-which runs on `ubuntu-latest`, builds a signed AAB, and uploads it to the Play
+which runs on the self-hosted `novaloop-incus-large` runner, builds a signed
-Store **internal** track. No GitHub Release is created.
+AAB, and uploads it to the Play Store **internal** track. No GitHub Release is
+created.
 
 **Promotion to production is always a manual step** in Play Console after an
 internal build has been validated — see "Promoting to production" below.
@@ -65,6 +66,34 @@ Use when retrying a failed run or rebuilding without moving tags.
      `HEALTHCHAT_CHANGES.md` lookup. Useful for ad-hoc test builds.
 3. Run. The AAB is built and uploaded exactly as the tag path would.
 
+## Runner and build tuning
+
+The job runs on `novaloop-incus-large` — an incus container with 4 cores and
+24 GiB, not a GitHub-hosted runner. Two consequences the workflow handles, and
+that anyone editing it needs to keep in mind:
+
+- **The image ships no Android SDK.** The `Set up Android SDK` step installs
+  it. Platform packages are minor-versioned, so `COMPILE_SDK = 37` means
+  `platforms;android-37.0`. Build-tools follows AGP's own default
+  (`SdkConstants.CURRENT_BUILD_TOOLS_VERSION`), not `BUILD_TOOLS_VERSION` in
+  `Versions.kt` — nothing sets `buildToolsVersion`, so bump it when AGP is
+  upgraded.
+- **`GRADLE_OPTS` is deliberately smaller than upstream's.** Upstream's
+  workflows use `-Xmx8g` with a 4 GB Kotlin daemon, sized for the 4-core/16GB
+  `ubuntu-latest` that *public* repos get; this repo is private, so its hosted
+  runners are 2-core/7GB and its container is 24 GiB. At upstream's numbers the
+  daemon plus the Kotlin daemon plus four parallel lint/R8 workers exceed the
+  limit and the daemon is SIGKILLed — the symptom is
+  `DaemonDisappearedException` with **no** `OutOfMemoryError` and no heap dump,
+  which is what an external kill looks like. `-Xmx6g` with a 2 GB Kotlin daemon
+  fits.
+
+The `Report runner capacity` step prints `free`, `nproc`, `df` and the enforced
+cgroup limit on every run, so a future failure can be read against the runner's
+actual capacity rather than guessed at. Note `free` reports the container's
+limit only when lxcfs virtualises `/proc/meminfo`; `/sys/fs/cgroup/memory.max`
+is the number the kernel enforces.
+
 ## Versioning (calendar scheme — differs from the old Classic repo)
 
 Element X uses calendar versioning, defined in