feat(acp): explain rejected inbound messages

[brow]

Problem

When someone without access messages a managed agent, the inbound author gate silently drops the event. The sender cannot tell whether the agent is ignoring them, unavailable, or restricted to its owner.

This complements #4053, which provides the internal-build owner-only enforcement.

Changes

crates/buzz-acp/src/inbound_denial.rs

• Add a deterministic owner-only explanation for rejected messages that explicitly mention the agent. Ordinary channel chatter remains silent.
• Reply in the triggering thread without a p-tag. The denied sender is deliberately not notified and sees the explanation only in-thread.
• Preserve silence for workflow traffic, other denial events, non-message kinds, and agents configured with respond-to=nobody.
• Prevent agent-to-agent denial loops through two independent guards: denial events carry no p-tag and are marked so other agents recognize and drop them.
• Bound explanation volume with per-author and per-channel controls. An agent explains itself at most a few hundred times in any ten-minute window; past that, new conversations get silence rather than an unbounded stream, and normal expiry restores it.

crates/buzz-acp/src/lib.rs

• Classify allowed, denied, and denied-with-explanation outcomes at the existing inbound author gate.
• Publish only for the denied-with-explanation outcome while continuing to drop every rejected event before filtering and queueing.

Tests

Added coverage:

• allowed_input_proceeds_when_every_denial_condition_is_true
• mentioned_input_suppresses_denial_when_false
• cooled_down_input_suppresses_denial_when_false
• kind_input_suppresses_denial_when_false
• channel_capacity_never_repeals_an_existing_channel_cap
• full_author_map_fails_closed_without_erasing_existing_cooldowns
• nobody_mode_suppresses_denial_for_an_otherwise_qualifying_message

Quality gates at tip 33c6e04e134c1b0d25a4d05da325d7118145b18d

• buzz-acp library tests: 676 passed, 0 failed.
• buzz-acp integration tests: 9 passed, 0 failed.
• Rust formatting, clippy with warnings denied, and the diff check passed.

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Originated from Buzz channel buzz-agent-control.