chore: add writing-e2e-tests skill (#24597)

Adds `yarn-project/.claude/skills/writing-e2e-tests/SKILL.md`: a skill
that gives an agent everything it needs to place, structure, and write a
robust e2e test with little guidance. Written against the v5-next e2e
layout (category directories with per-category READMEs and context
classes).

## What it covers

- **Step 0 decision ladder**: unit test → new expectation in an existing
test → new `it` in an existing suite → new file on an existing
context/harness → new standalone test.
- **Where to place the test**: recap of the category directories
(`automine/` via `AutomineTestContext`, `single-node/` via
`setupWithProver`/`setupBlockProducer`, `multi-node/` via
`MultiNodeTestContext` on the mock-gossip bus, `p2p/` via
`P2PNetworkTest` on real libp2p, plus `composed/`, `infra/`, `spartan/`,
`bench/`), the multi-node-vs-p2p decision rule, pointers to the
per-category READMEs as the authoritative reference, file naming
(`describe` matches path, header comment), automatic CI registration
through `bootstrap.sh` globs, `.parallel.test.ts` semantics, and
jest/bash timeout sync.
- **Setup reuse**: the three layers (category contexts → domain
harnesses → root `SetupOptions`), the standard suite file shape,
one-environment-per-file, preset spread order, guarded teardown.
- **Readability**: intent-only test bodies, the named-waiter surface
(`fixtures/wait_helpers.ts`, context waiters, `ChainMonitor`), shared
helpers and co-located `*_test_helpers.ts`, simulators with `afterEach`
checks, shared error constants.
- **Speed**: themes distilled from the recent
`test(e2e)`/`perf(e2e)`/`chore(e2e)` speedup PRs and their tracking
issues — avoid unneeded setup, genesis seeding over setup txs,
`BatchCall` batching over `Promise.all` overlap (with the
PXE-serialization caveat), warping dead waits with
`markProvenAndWarp`/`warpWithSequencersPaused` ("an honest wait beats a
flaky warp"), named timing profiles over ad-hoc cadences, and measuring
with the `testSpan` instrumentation + `track-e2e-times` skill before
optimizing.
- **Flakiness**: twelve golden rules synthesized from six months of
deflake PRs and the accumulated flaky-test gotchas (poll-don't-sleep via
named waiters, receipt-anchored assertions, mined ≠ checkpointed ≠
proven, `syncChainTip` tag awareness, pipelining timing margins +
`findSlotsWithProposers`, invariant-not-exact-value assertions, L1
account/nonce and port hygiene, freeze-L1-across-restarts, gossip-mesh
readiness via `runGossipScenario`, fee padding, determinism, honest
timeouts), plus `deflaker.sh` validation and `.test_patterns.yml` as a
last resort.
- Final pre-ship checklist.

All helper/API names cited were verified to exist on
`merge-train/spartan-v5` (`AutomineTestContext.setup`,
`setupWithProver`/`setupBlockProducer`,
`MOCK_GOSSIP_MULTI_VALIDATOR_OPTS`, `wait_helpers.ts` waiters,
`warpWithSequencersPaused`, `markProvenAndWarp`,
`testSpan`/`TEST_TIMING_FILE`, `runGossipScenario`,
`waitForP2PMeshConnectivity`, `findSlotsWithProposers`,
`getPaddedMaxFeesPerGas`, etc.). Genesis-prefill techniques that only
exist in still-open speedup PRs are described as themes to look for
rather than as concrete APIs.

(cherry picked from commit 4c2b6d8)

Author: AztecBot