Add a namespaced categorical-tag query to `skern skill list`

[Sophylax]

Summary

Add a generic, namespaced categorical-tag query to skern skill list. skern already filters on flat --tag (exact
match); this adds tags of the form category:value, so a caller can narrow a skill set by structured dimensions
without skern knowing what any category means. Fully domain-agnostic. (How untagged skills are treated and how
multiple flags combine are deliberate design questions — see below.)

Scope

• New repeatable --category category:value flag on skern skill list, with a matcher beside hasTag in
  internal/cli/skill_helpers.go.
• Category-agnostic: split each tag on the first : to derive its namespace; never enumerate or special-case
  known categories.
• Base match: for a requested category, a skill matches if it carries category:value. How a skill with no tag
  in that category is treated is a design question (below), not assumed here.
• Keep the existing --tag flag unchanged. Add --json contract coverage. Touches internal/cli/skill_list.go,
  internal/cli/skill_helpers.go, and tests.

Open design questions (settle before implementation)

Two semantics decisions shape the flag surface and the matcher:

1. Untagged / category-absent handling. When a skill has no tag in a requested category, does it match
("absent = applies to all") or not — and is that the default, or opt-in behind a flag (e.g. --include-untagged,
vs a stricter default)? This changes the base matcher.

2. Combination of multiple flags.

• Multiple values in the same category (e.g. --category lang:python --category lang:go) — OR within the category
  (match if the skill has any requested value)? This is the likely-needed narrowing case.
• Multiple distinct categories (e.g. --category lang:python --category topic:testing) — AND across categories
  (the skill must satisfy each)?
• A natural model is OR within a category, AND across categories. Confirm, and decide the flag shape: repeated
  --category cat:val, a comma list cat:a,b, or both.

Acceptance criteria

• skern skill list --category <…> --json applies the agreed combination + untagged-handling semantics.
• Untagged / category-absent skills are handled per the agreed rule (default or flag).
• Edge cases handled: tag with no colon, empty value, skill with zero tags.
• --json contract covered; passes the manual-test-verification gate.

Dependencies

None to build.