feat(cli): airs config — list/get/set/unset/path with source tracking

[cdot65]

Summary

No more hand-editing ~/.prisma-airs/config.json. airs config list shows every key with its effective value and source (env/file/default); set/unset write the file with full Zod validation; secrets are masked everywhere unless --reveal.

Changes

• src/cli/commands/config.ts (pure logic + registration), wired in program.ts
• loader.ts: inspectConfig() source tracking + PRISMA_AIRS_CONFIG_PATH override (whole CLI honors it; enables safe testing)
• Docs: configuration.md gains "Managing config from the CLI"

Testing

• 30 new tests (TDD, red-first); 720/720 total; lint + tsc + build clean
• Smoke: set→file written, get respects env>file cascade, invalid set exits 2 with Zod message, unknown key exits 2

Checklist

• TDD
• Gates green
• Docs + changeset (minor)

Closes #272