fix(proxy): align discovery container name

[vishr]

What this changes

• Rename the managed discovery container from onebox-proxy-discovery to onebox-discovery, matching the published image name.
• Keep backward-compatible teardown cleanup for the legacy name.
• Require exact container name plus the generated Compose project and service labels before any fallback force-removal, protecting unrelated same-named containers.
• Update Compose rendering and teardown ownership tests for current and legacy names.

Verification

• just check passes locally: module tidiness, formatting, vet, all Go tests, generated documentation verification, and the static site build.
• CLI-derived docs were regenerated; all 16 generated pages and the published artifact remain current with no diff.
• The manual docs/site contain no stale legacy container-name references.
• Astro reports zero errors, warnings, or hints.
• All five CI jobs pass, including Docker end-to-end and native CLI smoke tests on Linux, macOS, and Windows.
• The branch is based on current main, conflict-free, and mergeable.

Context

The original socketless discovery work is already on main through security-fork commit d6a79ab. This PR contains the focused container-name follow-up and its ownership-safe migration cleanup.