v0.6.21 — cleanup batch (AGENTS.md + empty state + placeholder audit)

v0.6.21 — Cleanup batch (AGENTS.md + empty state dedup + placeholder audit)

A small user-flagged cleanup release that closes three leftover
P2/P3 items. No new features, no schema bump; this is a
hotfix-shaped release that nudges a few long-standing paper cuts
and adds a regression test so the placeholder audit doesn't
drift again.

What's in the box

• AGENTS.md version drift fixed. Two places (the "30 秒
  判断题" header and the "Last updated" footer) said v0.6.14
  even though the project is at v0.6.20. Bumped both.
• /usage empty state no longer duplicates its hint. The
  usage.empty title used to re-state the "run pi with a real
  model" message that usage.empty.hint already said, so the
  user saw the actionable message twice. Now the title is a
  short descriptive label and the hint carries the actionable
  step alone.
• Placeholder parameter audit closed. v0.6.16 closed 8 of
  15 historical placeholder mismatches between en and zh but
  punted the rest. v0.6.21 closes the last 7:
  • 2 hardcoded-"1" in en singular keys (blockCount.one,
    packageCount.one) — both now use {n} like their
    *.other counterparts
  • 5 en-only plural-suffix placeholders ({s} / {es})
    — dropped in favor of always-plural English
  • 1 zh missing {n} (tools.subtitle) — added
• Regression test added. tests/i18n.test.ts now has a
  placeholder consistency across locales block that walks
  every key in both en and zh, asserts the placeholder sets
  match, and dumps the full offender list on failure.

Files touched (7)

Area	Files
Docs	AGENTS.md (2 version refs), CHANGELOG.md (v0.6.21 entry)
i18n	web/src/lib/i18n/dict.en.ts (5 strings), web/src/lib/i18n/dict.zh.ts (1 string)
Tests	web/tests/i18n.test.ts (1 new describe block)
Versions	package.json, web/package.json (both → 0.6.21)

Backlog after v0.6.21

• v0.6.22 — block-center avoidance (real A* grid router on
  top of the v0.6.20 route enum) + hooks 抽离
• per-direction palette (e.g. "all forward connections get this
  color") — deferred; per-edge is the v0.6.19 minimum