fix(persona): pack validate errors on skills the runtime would silently skip

[vedantggwp]

Implements the PF-2 remainder from PERSONA_PACK_SPEC.md section 16. No issue filed since
the spec documents this as remaining validate work, but happy to split one out if you
prefer tracking it that way. No duplicate found in open PRs.

The runtime silently skips any skill whose SKILL.md lacks name: or description:
(parse_frontmatter returns None, no fallback to the directory name). And a persona
skills: entry pointing at a directory that doesn't exist is silently dropped during
validation's collection pass. So buzz pack validate currently says Valid. for packs
that will quietly lose skills at deploy time. That's the worst kind of pass.

With this change, validate errors on:

• a persona skills: entry that doesn't resolve to a directory under skills/
• a skill directory missing SKILL.md
• SKILL.md with no frontmatter, or frontmatter that isn't valid YAML
• frontmatter missing name: or description: (or either being empty)

Unchanged: name-vs-directory mismatch stays a warning, same as before. Hooks path
verification (the other half of PF-2's remainder) is left for a follow-up to keep this
diff reviewable.

Tests cover each new error path plus one asserting the mismatch warning didn't get
promoted to an error.

DCO signed. AI-assisted, human-reviewed line by line.

What operators see

Real output from buzz pack validate built off this branch, against a pack with one bad
skills reference and one SKILL.md missing its description:

  ERROR: persona "t": skills entry "skills/nope" does not resolve to a directory under skills/
  WARN:  skill skills/code-review: name "code_review" differs from directory name "code-review"
  ERROR: skill skills/code-review: frontmatter is missing required field "description"

And the no-frontmatter case:

  ERROR: skill skills/code-review: SKILL.md has no frontmatter; the agent runtime silently skips skills without `name` and `description`

Each new error path has a matching unit test in validate.rs
(validate_pack_skill_ref_missing_dir_errors, validate_pack_skill_missing_name_errors,
validate_pack_skill_missing_description_errors, validate_pack_skill_no_frontmatter_errors),
and the pre-existing validate_pack_skill_name_mismatch_warns pins the mismatch case to
a warning so this change can't silently promote it.

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[Chessing234]

Good PF-2 validate tighten-up. Surfacing missing name:/description: and missing skill directories as hard validate errors matches what the runtime already silently drops — better to fail the pack than ship a persona that quietly loses skills. Would be nice to include a tiny fixture-based test case in the PR description (or point at the ones in the diff) so reviewers can see the exact error strings operators will hit.

[vedantggwp]

Good call. Added a "What operators see" section to the description with real output from a build off this branch, plus pointers to the tests covering each error path. Included the case where the naming warning shows up alongside the new errors, since that's the behaviour most worth eyeballing.