docs(agents): name the commands AGENTS.md's rules require

[siloteemu]

AGENTS.md states rules but, in several places, never names the command that
satisfies them. Someone following it exactly runs none of the behaviour tests it
mandates and fails CI checks with no way to learn from the file what to run.

What was wrong

• The prescribed gate could not run the tests the same file mandates.
  AGENTS.md requires user-observable behaviour changes to be covered by a
  Gherkin scenario under tests/e2e-cucumber/features/. Its quality gate
  prescribed cargo test --workspace --all-targets — but the e2e target sets
  test = false, so cargo test and cargo nextest skip every scenario
  silently and still report success. 17 feature files were reachable only via
  cargo xtask e2e, which the file never mentioned.
• The same test = false excludes the harness from the prescribed clippy
  run, so the e2e step and harness code went unlinted by the gate as written.
  CI covers it with a second, package-scoped invocation; the file did not.
• Two checks CI enforces hard were named in no contributor-facing document:
  the licence-header check and the third-party-notices staleness check. Their
  pre-commit hooks also fail differently when the underlying tool is absent
  locally — the notices hook passes --if-available and no-ops, so a green
  local run did not mean what CI means, while the licence-header hook is a
  plain language: system hook and aborts the run outright.

What changed

• The quality gate now lists prek run --all-files, the package-scoped clippy
  invocation for the e2e harness, and cargo xtask e2e, alongside the three
  commands it already had (kept verbatim).
• The scenario rule now names cargo xtask e2e at the point where the decision
  to write a scenario is made, including the filter form.
• The licence-header and third-party-notices checks are named with their direct
  CI-equivalent commands and install hints, and the file states how each hook
  behaves when its tool is missing — the notices hook no-ops, the licence-header
  hook aborts — so the difference between a locally green run and a CI-green run
  is explicit.
• AGENTS.md and CONTRIBUTING.md now cross-reference each other; previously
  neither mentioned the other.

Commands were sourced from the tool configuration and the CI workflow rather
than copied between documents, since for several tools the content did not
exist in either document.

Test plan

Documentation-only change; no behaviour is added, so no regression test applies.
Every command added to the file was verified by executing it in a clean
checkout:

Command	Result
cargo xtask e2e	ran 78 scenarios, 74 passed, 2 unexpected failures — pre-existing timing-sensitive bench throughput scenarios on a loaded host, unrelated to this change. The harness reconciles expected xfails separately from passes, so the two counts do not sum to the total.
prek run --all-files	exit 0, 14 hooks passed
hawkeye check --config licenserc.toml	exit 0, 230 files
cargo xtask tpn --check	exit 0

prek run --all-files also passes on the edited files.

Checklist

• If this PR fixes a bug, searched tests/e2e-cucumber/expectations.toml for the fixed ticket ID and removed/narrowed any now-stale xfail rows. — Not applicable: documentation-only, no code behaviour changes, so no xfail row can become stale. Confirmed expectations.toml is untouched by this PR.

Notes for reviewers

• The second clippy line (-p e2e-cucumber --test e2e) addresses the same
  test = false root cause as the e2e gap and comes from the CI workflow. It is
  adjacent to the originally reported problem rather than part of it — happy to
  drop it if you would rather keep this change narrower.
• The pre-existing cargo clippy --workspace --all-targets line lacks --locked
  while CI uses it; the newly added clippy line matches CI byte-for-byte. Left
  the pre-existing line alone to keep this change scoped, but happy to align it.

[jussielo-amd]

Reviewed. Docs-only change (AGENTS.md, CONTRIBUTING.md), no runtime/CLI-facing impact.

Correctness: All 11 new factual claims (e2e test = false quirk, exact CI clippy invocation, cargo xtask e2e, license-header hook aborting vs. third-party-notices hook silently no-oping, etc.) were independently verified against the repo — all accurate. Fixes the four gaps it describes; a few other "rule with no named command" spots remain elsewhere in AGENTS.md (§3/§13, §6, §8, §12) — same defect class, out of scope here, good follow-up candidate.

Note: you already flagged it yourself, but the new e2e-cucumber clippy line uses --locked (matching CI) while the adjacent pre-existing workspace clippy line still doesn't, even though CI's version has it too — worth aligning whenever convenient.

UX: minor flow nits only — the new §3 paragraph sits between the topic sentence and the bullets it explains (slightly breaks the antecedent), and the AGENTS.md cross-reference in CONTRIBUTING.md lands mid-setup-section without a transition. Neither is misleading, just placement polish.

No blocking issues, CI green. Approving.