feat(skills): add `skills list` and bundle `glab-stack` skill

Description

Extends the experimental glab skills command and adds a new bundled skill for glab stack.

glab skills list renders the bundled agent skills in a table that wraps to the actual terminal width. Both list and install share a new internal/commands/skills/bundled package that owns the embedded SKILL.md assets and parses YAML frontmatter as the single source of truth for skill metadata.

glab skills install now accepts an optional positional <name>. With no argument it installs every bundled skill, preserving today's behavior; per-skill errors are aggregated via errors.Join so one failure does not abort the rest.

glab-stack skill guides agents through glab stack: which subcommands hang on interactive TUIs/prompts, the difference between save and amend, navigation patterns, and the bottom-to-top merge rule.

Related Issues

Follow-up to the experimental skills command introduced in v1.84.0.

How has this been tested?

• New unit tests in internal/commands/skills/bundled, install, and list.
• Verified manually:
  • glab skills list lists glab and glab-stack with wrapped descriptions on a TTY.
  • glab skills install with no args installs both bundled skills.
  • glab skills install glab-stack --path /tmp/foo installs just that skill.
  • glab skills install bogus returns a friendly error pointing to glab skills list.

Screenshots (if appropriate):

N/A