Linux CI smoke workflow

[0xLeif]

Part of GOAL.md workstream C. Also settles the README's Linux claim with evidence instead of aspiration.

Add an ubuntu (GitHub-hosted) workflow: swift build, swift test, and a core-commands smoke (get/set/watch-bounded/dump/reset) using an isolated state dir.

Acceptance criteria

• Workflow runs on pull_request and main pushes
• Builds with a Swift.org 6.x toolchain container or setup action
• Smoke runs against a temp state root, not $HOME
• README CI table updated

[0xLeif]

Shipped in PR #2 (aps 0.2.0, merge commit 62198af). Verified in review: gates green, behavior confirmed locally.