feat(cli): add 'buzz agents set-directory' kind:10100 directory publisher

[kevine-pixel]

Summary

• Adds buzz agents set-directory — a publisher for the kind:10100 agent directory record. Addresses the publisher gap in #5681 mention gate requires kind:10100 fields that nothing publishes — relay-hosted agents are un-mentionable #5869: since fix(desktop): enforce agent mention authorization at send boundaries #5681, the desktop admits a non-managed agent into mention autocomplete/send only when its 10100 content carries channel_ids and respond_to, but the only existing 10100 publisher (buzz channels set-add-policy) writes just {"channel_add_policy": …} — and, because kind:10100 is replaceable, clobbers any richer record. Relay-hosted agents were therefore structurally un-mentionable.
• The command read-merges the existing record so unrelated fields — notably channel_add_policy, which the relay's handle_agent_profile ingest requires — survive a republish.
• --name defaults from the identity's own kind:0 display_name/name; --channels-from-membership derives channel_ids from the identity's kind:39002 memberships; --no-channels supports DM-only agents; --respond-to owner-only|allowlist|anyone with --allow <PUBKEY> for allowlists (validated 64-hex, normalized lowercase).
• The NIP-OA auth tag is injected by the existing sign_event path, unchanged.

Validation

• Unit tests: merge preserves unrelated fields (channel_add_policy, unknown keys) and overwrites stale directory fields without touching a customized agent_type; subcommand stability guards updated.
• cargo clippy / cargo fmt clean; full buzz-cli test suite passes (345).
• Live-validated: 24 production relay-hosted agents republished their records with this command (each signing with its own identity + auth tag); the desktop fix(desktop): enforce agent mention authorization at send boundaries #5681 mention gate then admitted them, confirmed end-to-end by the community owner. The 3 DM-only agents published --no-channels records.

Related: #5869 (publisher gap — this PR), #5870 (directory inspection UI, separate).

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