check: 'name:' is required on every entity, but commands are named by file

[imarios]

Problem

checkName in src/core/frontmatter.ts warns missing required field 'name'
for any entity whose frontmatter omits name:. It applies the same rule to
skills, agents, and commands.

Claude Code slash commands are named by filename, not frontmatter — a
commands/deploy.md with only description: is completely valid and is how
most commands in the wild are written. skilltree's own scaffolder agrees that
the name is path-derived: entityNameFromPath() in src/core/entity-type.ts
falls back to the filename stem precisely because "commands and many agents are
named by file, not by frontmatter".

So check warns about a correct file, and --strict exits 1 on it.

Relationship to #159 / #160

Same class of bug: the linter being stricter than the runtime it lints for.
Those two were hard errors on unknown keys and are fixed in #162; this one is a
warning, so it only bites under --strict — which is why it wasn't reported
alongside them. It's the remaining instance of the pattern.

Why it's now easy to fix

#162 threads the resolved entityType into validateFrontmatter. Before that
change the validator had no way to know it was looking at a command. Now it
does, and checkName is the one check that should consume it but doesn't:

// src/core/frontmatter.ts — validateFrontmatter
...checkName(fm, context.entityName),   // <- entityType is in scope, unused here

Proposed fix

Make name: optional for type: command — either skip the missing-field
warning entirely, or keep validating it only when present (so a name: that
disagrees with the filename is still reported, which is the genuinely useful
half of this check).

Worth deciding the same question for agents, where name: is conventional but
also path-derivable.

Found while working on #159/#160.