feat(cli): support visibility on channels update

[tlongwell-block]

What

Add --visibility <open|private> to buzz channels update, so a channel's visibility can be flipped after creation from the CLI. Previously visibility was a create-time-only field: channels create took --visibility, but channels update only handled name/description/ttl.

Why it's small

The plumbing below the CLI already existed:

• SDK — buzz_sdk::build_update_channel already accepts a visibility arg, validates open|private, and emits the ["visibility", v] tag on the kind:9002 edit-metadata event (crates/buzz-sdk/src/builders.rs:572). Already covered by SDK tests (:2069, :2086).
• Relay — side_effects.rs already validates the tag value, gates it behind owner/admin auth, applies the DB update, invalidates the visibility/accessible-channel caches, and emits a visibility_changed event (:382-397, :424-437, :1075-1107).

The CLI just hardcoded None for that arg and exposed no flag. This PR wires the flag through — no SDK or relay changes.

Change

• lib.rs: add visibility: Option<ChannelVisibility> to the Update clap variant (value_enum, so invalid values are rejected at parse time with [possible values: open, private] — matching channels create).
• channels.rs: thread it into cmd_update_channel, include it in the no-op guard + error message, and pass it to build_update_channel.

No public alias — held to open|private to match create, the SDK, and relay validation (one vocabulary).

Testing

• cargo test -p buzz-cli -p buzz-sdk — green (135 + 205).
• cargo clippy -p buzz-cli — clean.
• cargo fmt -p buzz-cli --check — clean.
• Live smoke against a real channel (f5bf2e54-...), each flip confirmed via the stored kind:9002 visibility tag on the relay:

buzz channels update --channel <uuid> --visibility private   # -> accepted; relay stores ["visibility","private"]
buzz channels update --channel <uuid> --visibility open      # -> accepted; relay stores ["visibility","open"]
buzz channels update --channel <uuid> --visibility public    # -> rejected at parse: [possible values: open, private]

Review

Planned and diff-reviewed with Wren; independently confirmed not already wired by Max.