docs: lead with the one check nothing else does, and cut the install to one option

[sjh9714]

What was wrong

The README opened with a claim about the world (AI coding tools open pull requests faster than anyone can read them) rather than about the reader. It
then told a story lifted from the demo fixture, and the first action it asked
for was running a demo of a synthetic pull request. Nobody runs a demo of a
tool they have not decided they need.

What people receiving pull requests actually want

They want the queue to shrink. peakoss/anti-slop
has 751 stars for "detects and automatically closes low-quality and AI slop
PRs", written by a Coolify maintainer taking 120+ a month.

Its own users then file: #19
closed pull requests cannot be reopened, #16
the spam-username rule flags legitimate people, #13
the failure message does not say which check failed.

So the order is removal first, accuracy and reasons second. MergeWarden
refuses removal on purpose and only does the second. This pull request does not
close that gap and does not claim to.

What it does instead

It puts first the one thing that appears uncontested: a pull request that edits
the files instructing your coding agents. anti-slop's 34 rules do not cover it,
and searching for repositories doing the same turned up nothing above 30 stars.

Verified before the copy was written, not after. A pull request titled
"docs: tidy up the contributor notes", whose entire diff is one added line in
CLAUDE.md, produces exactly the comment the README quotes:

sjh9714/agent-gate-install-smoke-20260617#22

That run also reports Agent detected: no, so the check does not depend on the
pull request being agent-authored. The README says so.

The 3.9% figure stays in the README. It is not a daily event, and inflating
it would repeat a mistake this project has already made.

Install: four options to one

Three were doing nothing at all:

Removed	Why it was inert
github-token	action.yml already defaults it to ${{ github.token }}
mode: warn	packages/core/src/config/schema.ts:431 already defaults to "warn"
fail-on-block: false	packages/action/src/run.ts:523 only fires on decision === "block", which warn mode never produces

templates/mergewarden-observe.yml loses its with: block entirely.

comment's default is deliberately not changed. Making auto the default
would warn on any workflow granting only pull-requests: read, which is the
trap fixed in #180.

The reduced snippet is the one that produced the run linked above, so it is
verified rather than assumed.

Also

• triage's token requirement moves above the command, with a link to create
  one and the fact that no scopes are needed.
• README.zh-CN.md gets the new opening plus an explicit note that the rest is
  still the older structure.
• documentation.test.ts asserted the old heading order, so it asserts the new
  one. The 300-line and 1,500-word limits are unchanged: 190 lines, 1,093 words.

Verification

pnpm build && pnpm test && pnpm typecheck && pnpm lint && pnpm format:check,
all green (461 tests). No em-dashes in any user-facing document.