feat: add installable multi-platform CLI releases

[robinbraemer]

Intent

Deliver the captain-approved Akua CLI distribution, README, and release slice using self-contained GitHub Release artifacts plus Homebrew. The canonical executable remains akua; npm publication is intentionally excluded.

Merge order and unresolved dependency

• Hold this PR behind feat(skills): add canonical Akua agent skill artifact #20. PR feat(skills): add canonical Akua agent skill artifact #20 owns the canonical skill source at skills/akua/SKILL.md with skill name akua and must merge before this PR.
• This PR intentionally contains no skill source files or skill-content validation. It retains only README and AGENTS ownership references to the canonical path.
• The Homebrew receiver/formula migration and existing v0.8.25 to CLI v0.8.0 version-ordering decision remain unresolved. Do not merge, release, publish, or mutate the tap until the captain approves that coordinated path.

What changed

• Add self-contained akua archives for macOS arm64/x64, glibc Linux arm64/x64, and Windows x64.
• Generate stable artifact names, executable-only archives, SHA-256 files, release manifests, and exact Homebrew artifact/checksum mappings.
• Add native install smoke coverage for akua --version, akua --help, and akua commands --limit 1.
• Make release uploads immutable and resumable, verify published assets before tap handoff, pin tag commits, and avoid persisted publish credentials.
• Rewrite install, authentication/config, adaptive output, OpenAPI generation, upgrade, platform, and release documentation.
• Preserve regressions for canonical executable identity, auth unknown-key preservation, OpenAPI update scope/idempotence, deterministic operationId generation, and agent/provider detection.

Validation

• bun test test/docs.test.ts: 3 passed.
• mise run check: 70 passed, generated drift and build green.
• no-mistakes v1.35.0 previously validated the release workflow, five-target packaging, native install transcript, lint, push, PR, and CI; subsequent changes are reference-only.

No tag, GitHub Release, package publication, tap mutation, merge, or irreversible publication was performed.