docs(education): fix first-skill capability examples

[kushin25]

Summary

Corrects invalid beginner-guide capability examples and the command used to launch the skill-authoring guide.

Changes

• replaces three undefined capability:* examples with documented capability labels;
• changes both /write-skill references to /magpie-write-skill.

Related issue

Closes #974

Validation

• git diff --check — passed
• verified each of the five displayed capability labels is listed in docs/labels-and-capabilities.md
• verified both command references use /magpie-write-skill
• uvx --from 'prek>=0.4.9' prek run --all-files — could not complete locally because the available uv 0.11.6 is below this repository's declared minimum >=0.11.8; the failure occurred in project tooling, before the uv-backed validation hooks could run.

Notes

This small documentation patch was drafted with AI assistance and reviewed by its submitter. It changes only the file named in the issue.

[justinmclean]

Verified: all five labels in the new text (triage, authoring, review, fix, intake) appear in the Axis-1 table in docs/labels-and-capabilities.md; the three removed ones (security, release, contributor-growth) exist nowhere. skills/write-skill/SKILL.md declares name: magpie-write-skill, so /magpie-write-skill is right. Thanks for fixing this!