feat(agents): open agent imports from the CLI

[bartlomein]

Why

Beekeep.sh is a public, reviewed registry of Buzz agents. Its CLI can download and verify an approved agent snapshot, but users still need a safe handoff into Buzz Desktop.

This adds that handoff. Beekeep and other native tools can ask Buzz to open its existing import review without silently creating an agent. The user remains in control and must explicitly click Import.

What changed

• Add buzz agents import --file <snapshot> as a local-only CLI command.
• Add a versioned buzz://agent-import?v=1&file=... Desktop deep link.
• Focus Buzz, navigate to My Agents, and open the existing snapshot review automatically.
• Add bounded and expandable instruction review, validation, tests, and integration documentation.

Safety

• The handoff accepts only absolute local .agent.json or .agent.png files within documented size limits.
• Buzz Desktop independently validates the snapshot before showing it.
• Opening the link never creates an agent; creation still requires an explicit Import click.

Screenshots

Testing

• cargo test -p buzz-cli
• ./bin/just desktop-tauri-test
• Focused Playwright agent-import deep-link test
• Manual v=1 deep-link review in the dev app
• ./bin/just ci and git diff --check