Keep the landing intact when a plugin warns at startup

[TheGreatAxios]

A startup plugin warning went out through appendStreamRow, whose first statement is clearLandingMark — so one warning took the whole landing hero with it: the mark, the hints beside it, the composition. A first run in any repo with an unresolved skill showed no landing at all.

CL-5618 routed MCP and hook failures through surfaceStartupNotice for exactly this reason; its comment says so outright. Plugin diagnostics were the path it missed.

What changed

• startupPluginNotices ride the notice strip instead of the transcript
• The internal command gutter label is gone from system rows — plumbing, not a word an operator would use for what they are reading
• formatPluginWarningsSummary dedupes: one skill referenced by three plugins produced three warnings, so seven missing skills printed as nine entries and the count disagreed with the list

Scope

The bug half of CL-5718, split out so it lands now. The composition work — snow, changelog-on-fresh-install, the ranked notice register, short-terminal degradation — continues separately against the mockups on the ticket. Three meta: "palette" labels remain and belong with that pass.

[linear-code]

CL-5718

[TheGreatAxios]

Closing in favour of cl-5718-landing-hero-impl, which does this properly.

I patched the one flush site. That branch found that systemRow is the only appendStreamRow caller in runner.ts — feeding all ten of its call sites — and fixed the wiring layer instead, behind a named seam (startup-notices.ts, flushStartupNotices) so "a startup diagnostic is never a transcript row" has one owner rather than a per-call-site convention.

That is the right shape and the same lesson as the grep/secret-scrub finding: a cross-cutting constraint applied at call sites is one forgotten call site away from being false. It also removes the three meta: "palette" labels this PR left behind and absorbs the falling snow from #380.