bug: stale error message says flat SKILL.md repos are not supported after #232 fix

[christso]

Bug

After #232 / #234 landed support for auto-wrapping flat SKILL.md repos, the error message in plugin skills add --from is still outdated.

File: src/cli/commands/plugin-skills.ts line ~377

Current message:

Skill 'X' not found in plugin 'Y'. The plugin may not use the allagents skills/ directory structure (flat SKILL.md repos from the npx skills ecosystem are not yet supported). (see GitHub for details)

Problem: The parenthetical "flat SKILL.md repos from the npx skills ecosystem are not yet supported" is no longer true — #234 added support for multi-skill flat repos (subdirectories with SKILL.md). This message will confuse users who installed a compatible flat repo but made a typo in the skill name.

Fix

Update the error message to:

1. Remove the "not yet supported" claim
2. Give actionable guidance: list available skills from the plugin instead (or point users to allagents plugin skills list)

Suggested replacement:

Skill 'X' not found in plugin 'Y'. Available skills: <list>

Tip: run `allagents skills list` to see all installed skills.

Note: The existing code already has skillNames in scope, so the list can be shown without additional logic.