lint: stop W2 switching itself off, and see an asset name nobody declared

[inhuman]

What

Two linter reports from an outside review, both reproduced by a failing test
before anything was changed. The format did not change — EngineVersion
stays 2.2.4, no skill file is affected.

1. W2 switched itself off offline

The rule makes two checks and only one needs the installation:

check	needs
the program uses a server the skill did not declare	the skill alone
a declared server is not registered	Facts.ServerNames

Interleaved, the registry was asked for on the first declared server — which
in any healthy skill comes first — and a missing one returned out of the whole
walk. So without a registry the rule did not degrade partially; it turned itself
off, and the undeclared server further down was never reached.

That is what let a live catalogue report "0 errors" while carrying two:
steps called a server their skill had not declared, the radius was never handed
over, and 21 turns answered "no such pod" about live pods.

Now two passes — the skill against itself, then the skill against the
installation — which makes the dependency structural instead of a comment. The
skip still fires (a report must distinguish "part of a rule did not run" from
"the rule did not run"), but only when pass two had something to check.

The patch that came with the report would have introduced false findings. It
handed the loop an empty registry and continued, so on the second declared
server !known[srv] is true and every existing server is reported as "not
registered" — a report full of findings about live servers, which is worse than
the silence it replaces. A test pins that now.

The order of the steps in the fixture is the test: put the undeclared server
first and it passes on the broken code too.

2. W19 / W20 — asset names

A reference to an asset the skill does not declare was caught by nothing.

The engine expands an unknown asset to an empty string by contract — a
marker left in an instruction would be read by the model as part of it — and
that contract is right. The price nobody was paying: a typo is indistinguishable
from an empty asset, and what fails is whatever stood next to it. The call is
rejected by the server for a missing argument, and the error names the
argument rather than the substitution a floor above.

• W19 (error): every {{asset:name}} and every {from: "asset:name"} names
  a declared asset. Both forms, because both are used — one puts content in the
  text for the model, the other passes it into an argument past the context. The
  finding lists the declared names, since a typo is recognised by comparison.
• W20 (warn): an asset declared and never referenced. Clutter rather than a
  failure, and worth having because assets outlive the steps that used them.

Both are purely static — declarations and references live in one document, no
registry of anything involved.

One thing the report did not mention: workflowAssets returned early when the
asset map was empty, so a skill with no assets that still referenced one
would have been missed. The check now runs before that exit.

Also documented

Rule.Needs read as "the rule cannot run without this", while for W2 it is one
of two checks. It now says so on the type, and the partially-dependent rules say
which half needs what — including W12, which the report rightly notes is
entirely dependent on a live tool listing and does not run offline at all. A
CI report of "0 warnings" was giving false confidence about it.

Testing

• go vet ./... clean
• go test ./... -count=1 all green
• go test ./... -race all green
• Both example modules vet and test green
• Every finding reproduced first; the new fixture is registered and the
  catalogue/README guards updated (27 → 29 rules everywhere)

Format & API compatibility

• The format did not change; EngineVersion stays 2.2.4 and CHANGELOG.md
  (which documents the format) is untouched
• Go API unchanged
• Behaviour change for linter users: skills that were reported clean may
  now show W2 findings offline, and W19/W20 are new — that is the point of
  the change

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