chore: add writing-e2e-tests skill

[AztecBot]

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.