docs: mind the build machine and keep full test runs on CI

RULES.md gains "The machine you build on": check memory, load and running
builds first, cap Cargo jobs and test threads, run at low priority, one heavy
job at a time, stop when the person says the machine is slow, and clean up
scratch targets. Locally only the test targets a change touches run; the full
suite runs on GitHub before a release. AGENTS.md, CONTRIBUTING.md and the PR
checklist say the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,7 +17,7 @@
 ## Checklist
 
 - [ ] Tests first; new code is covered, error paths included
-- [ ] `cargo fmt --all`, `cargo clippy --all-targets` and `cargo test` pass
+- [ ] `cargo fmt --all`, `cargo clippy --all-targets` and the affected `cargo test --test` targets pass
 - [ ] Follows [RULES.md](../RULES.md) (no overflow, data safety, one undo step per tool call)
 - [ ] Docs, tool descriptions and `PROGRESS.md` updated where needed
 - [ ] Conventional commit messages

AGENTS.md

@@ -11,11 +11,19 @@ you is accountable for.
 cargo build --release
 cargo fmt --all
 cargo clippy --all-targets
-cargo test --no-fail-fast                       # prefer one test: cargo test --test mcp name
+cargo test --test incremental                   # only the targets your change touches
+cargo test --lib editor::changes                # unit tests of one module
 cargo check --lib --target wasm32-unknown-unknown
 ```
 
-`tests/update.rs` cannot run on Windows (it needs elevation); that failure is known.
+Do not run the full suite locally: it builds dozens of test binaries. It runs on GitHub before
+each release (`.github/workflows/test.yml`). `tests/update.rs` cannot run on Windows outside CI
+(it needs elevation); that failure is known.
+
+Builds run on the person's own machine, so mind its resources (RULES.md, "The machine you build
+on"): check free memory and running builds first, cap jobs at no more than half the cores
+(`cargo test -j 8 -- --test-threads=8` on 16 cores), run at low priority, run one heavy job at a
+time, and stop at once if they say it is slow.
 
 ## Running the app safely
 

CONTRIBUTING.md

@@ -69,10 +69,14 @@ fetches from a public git repository at the revision pinned in `Cargo.toml`.
 ```sh
 cargo fmt --all
 cargo clippy --all-targets
-cargo test --no-fail-fast
+cargo test --test <target>                            # the targets your change touches
 cargo check --lib --target wasm32-unknown-unknown     # the web build
 ```
 
+Each file in `tests/` is its own binary linking the whole app, so the full suite is slow and heavy
+on a desktop. It runs on GitHub instead: before every release (`.github/workflows/test.yml`), or
+on demand with `gh workflow run test.yml -f ref=<branch>`.
+
 Keep the tree free of the third-party branding patterns listed in `.debrand`.
 
 ### Tests

RULES.md

@@ -40,6 +40,28 @@ PaperCraft stores people's work. Treat that as the top priority.
   says so.
 * Destructive operations are undoable or confirmed.
 
+## The machine you build on
+
+A full build links dozens of test binaries and can take every core and most of the memory of a
+desktop. The person is still using that machine, often with PaperCraft open.
+
+* **Look before you start a heavy job.** Check free memory, CPU load and what is already running
+  (the person's app, another build, other agents) before a build, a test run or clippy. If memory
+  is short or another build is running, wait or ask.
+* **Leave room.** Cap Cargo at no more than half the logical cores (`-j 8` on a 16-core machine,
+  or `CARGO_BUILD_JOBS`) and the test harness too (`-- --test-threads=8`); go lower when memory is
+  tight, since linking is the peak. Run heavy jobs at low priority (`nice` on Unix, Below Normal
+  on Windows).
+* **One heavy job at a time.** Never run two builds side by side; each target folder is a full
+  build's worth of compiling and linking. Parallel agents share one build slot.
+* **Run only what your change touches**: `cargo check` before `cargo build`, and the test
+  targets that cover the change (`cargo test --test incremental`). The full suite runs on GitHub
+  before a release (`.github/workflows/test.yml`), not on a desktop.
+* **Stop at once when the person says the machine is slow**, then check that no compiler, linker
+  or test process of yours is left over.
+* **Clean up.** Delete scratch target folders and worktrees when the work is merged; they grow to
+  tens of gigabytes.
+
 ## UI
 
 * **Use the shared kit.** Controls, spacing, radii and colours come from `craft-ui`,