docs(ai): add acceptance test guide and skill

[ajsutton]

Summary

• Add docs/ai/acceptance-tests.md — full guide for building deps and running acceptance tests locally
• Add .claude/skills/run-acceptance-tests/SKILL.md — actionable skill for AI agents
• Refactor op-acceptance-tests/justfile:
  • Extract build-deps target from acceptance-test
  • Add just test *args target for running individual tests with automatic dep building
  • Remove git submodule update from local builds (can revert in-progress submodule work)
  • Always rebuild cannon prestates instead of conditional skip
• Reference new doc from AGENTS.md and docs/ai/dev-workflow.md
• Recommend RUST_JIT_BUILD=1 for local runs so op-reth and other Rust binaries build on demand

Test plan

• Ran RUST_JIT_BUILD=1 cd op-acceptance-tests && just test -run TestUnsafeGapFillAfterSafeReorg ./op-acceptance-tests/tests/sync/elsync/reorg/ — passed in ~79s
• Verify just acceptance-test still works for full suite
• Verify just acceptance-test base works for gated subset

🤖 Generated with Claude Code

[sebastianst]

I ran the new just target locally to confirm it work, with mise active: RUST_JIT_BUILD=1 just test -run TestUnsafeGapFillAfterSafeReorg ./op-acceptance-tests/tests/sync/elsync/reorg/

[wwared]

I think it would be good to ensure some just target or workflow still runs git submodule update --init --recursive --single-branch -j 8 because not all PRs change things in the submodules, and remembering to keep them updated can lead to weird test errors similarly to outdated contract or cannon artifacts. It might be enough to add a new just target update-deps (or similar) that does this and mentioning it in the ai doc/skill

[ajsutton]

We don't generally have just targets to do git updates and I think updating submodules belongs with that - it's just done as part of your git workflow. If the submodules aren't up to date, it shows up as the git checkout not being clean so AI should sort that out itself and humans usually can as well.