feat(relay): auto-add relay owner to newly created channels

[wilsonwang0713]

Problem

When an agent (or another member) creates a channel, only the creator becomes a member. The relay owner does not see the new channel in their client until they manually join — and for private channels they cannot self-join at all (kind:9021 is open-channel only). This is especially confusing with agent-created channels: the agent reports "created", but the owner's channel list stays empty.

Change

Add BUZZ_AUTO_ADD_OWNER_TO_CHANNELS (default off). When enabled and RELAY_OWNER_PUBKEY is set, the relay owner is automatically enrolled as an admin member of every channel they did not create.

• Runs in handle_create_group, before NIP-29 discovery emission, so the kind:39002 members roster already includes the owner.
• Emits a member-added notification so the owner's client subscribes in real time (mirrors the creator notification).
• Works for private channels too — direct membership add, not a self-join.
• Idempotent and non-fatal: the channel and creator membership are already committed if the owner add fails.
• Skips when the owner is the creator, or when the owner pubkey is unset/invalid.

Testing

• New config unit test auto_add_owner_to_channels_defaults_off_and_opts_in (default off; opt-in on).
• cargo fmt, cargo clippy -p buzz-relay, and cargo build -p buzz-relay pass.
• Verified end-to-end against a deploy/compose relay: an agent-created channel appears for the owner within the same turn, no manual join.

Notes

Opt-in and default-off, so existing single- and multi-tenant deployments are unaffected unless they explicitly enable it.