feat(skills): add canonical Akua agent skill

[robinbraemer]

Intent

Add the CLI-owned, versioned Akua agent skill at skills/akua/SKILL.md. Preserve one-way ownership: the CLI repository is canonical, and any downstream import remains a separate future change.

Scope

• Canonical Akua skill source and deterministic package metadata.
• Release Please extra-file wiring for the package metadata.
• Focused validation of canonical identity, valid Agent Skills frontmatter, deterministic packaging, provenance, and token-safe authentication guidance.
• No README, documentation, release-workflow, binary-distribution, Homebrew, npm, or AGENTS.md changes.

Safety

Agents must not put authentication tokens in tool arguments, process argv, transcripts, or logs. When setup is requested, they direct a human to run akua auth login --token <token> privately and locally; agents can then use akua auth status.

Validation

• Pipeline run 01KXGBEPS4F1X3BMDZAFBD46TY completed review, tests, lint, push, and PR validation; the documentation step was intentionally skipped to preserve the approved narrow scope.
• Focused skill/release metadata tests and the full Bun suite passed.
• Two independently generated skill archives matched byte-for-byte.
• Exact PR head: 0eb081bdbf9c51315adf1d59ec037e914862ec1d.
• PR checks: 1 passed, 0 failed.

No merge, tag, release, publish, or downstream sync was performed.