Snapshot v2: carry skills (agent_snapshot.rs deferred item) — fixtures offered

`agent_snapshot.rs` specs skills as a deferred item for the snapshot format. We'd like to pick that discussion back up.

Context: [hivepack](https://github.com/syncreus/hivepack) converts Claude Code agent definitions into Buzz personas and bundles each agent's skills directory into the pack. That makes it a natural round-trip test case for snapshot v2: export a snapshot of an agent that has bundled skills, import it on a second machine, and the agent still has its skills.

**Offer:** we can contribute test fixtures (real packs with skills of varying shapes — plain markdown, scripts, nested resources) and a draft of the zip layout from hivepack's pack format, if the maintainers want to restart the format discussion. No strong opinions on the final layout — mainly we want the capability and are willing to do legwork.

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