docs(agents): record the agent working rules — slot dates, ≤5 concurrent agents, browser-verify before merge

[hotlong]

Fixes #132

Writes four maintainer instructions into AGENTS.md, where an agent will read them, instead of leaving them in dispatch messages. AGENTS.md is the only file touched.

What changed

Publishing cadence gains one sentence making the existing rule explicit for an agent that writes a post: its date is the slot it will occupy, it never stacks onto a day another post already holds, and it merges on that day. The surrounding bullets already carry "two slots a week" and the merge-day rule, so the sentence points at them rather than restating them.

New ## Working in this repo with agents, placed after Publishing cadence (and its two ### subsections) and before Localization, with four bullets:

• the concurrency ceiling of five agent worktrees (maintainer, 2026-09-02);
• browser verification at the six real viewports before a content or visual PR merges — scroll lazy images into view, look at the screenshots, render new SVGs at native size, plus the two dark-on-dark SVG traps;
• a visual defect is work, not a note (maintainer, 2026-08-26);
• merge time is publish time; never batch-merge; date is rewritten only by an explicit maintainer ruling recorded on the card.

Two deliberate calls, for the reviewer

The SVG clause names two traps, not one. The card specified the pattern where a fill presentation attribute is shadowed by a stylesheet class (#125 / #133). Finding #140 measured a second one on the same surface: a text node carrying neither a class nor an inline colour inherits the house text{fill:#173a36} default, so text on a dark card renders dark-on-dark that way too. Both reach the reader identically and both are invisible to every gate, so the clause covers both in one sentence rather than describing a check that catches half the cases.

Line budget. The card asked for ≤ 12 new lines; the diff is 14 insertions. Twelve of those are content lines — the other two are the blank separators a markdown ## heading requires and cannot be written without. Under that reading the change sits exactly at the ceiling, and folding #140 in was what took it from 11 content lines to 12. Flagging the arithmetic rather than asking you to take it on trust: if you read the budget as raw inserted lines, the #140 clause is the one line to drop.

Gates

Run once through the shared verify lock at head a1dd0a8, each gate's own verdict line quoted:

Gate	Verdict line
pnpm content:lint	✓ content lint passed (334 files checked)
pnpm check	- 0 errors / - 0 warnings / - 0 hints
pnpm build	[build] 866 page(s) built in 76.28s · [build] Complete!
pnpm seo:smoke	SEO smoke test passed (865 HTML pages checked)

Wrapper: os-verify-lock: VERDICT command-exit 0 · held the lock 97s (1m37s) · waited 0s. git status --porcelain empty afterwards — gen:zh-hant produced no changes.

AGENTS.md is a governed surface, so this stays a draft for a human merge; review requested from @os-zhuang.

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