Add built-in /compact slash command to the native agent

[rtfeldman]

Adds an always-available /compact slash command to Zed's native agent that forces a summary-based context compaction of the current conversation, regardless of token usage. This reuses the agent's existing compaction logic (always the summary strategy, never provider-native), bypassing the automatic-compaction token threshold and minimum-context-window guard since the user explicitly asked to compact. The command lives entirely in NativeAgent/NativeAgentConnection, so external (ACP) agents are untouched and keep their own /compact implementations.

The command is gated behind the handoff feature flag, matching the gate on automatic compaction, so it's only advertised and dispatched when that flag is enabled. When the flag is off, typing /compact is treated as an ordinary prompt.

Because the agent panel always renders the typed /compact as a user message, the manual compaction records a matching zero-content user message in the model thread (skipped when building model requests) so that rewinding/truncating that message works instead of erroring, while keeping the command itself out of the model's context.

The slash-command popup now groups commands into separate sections by source — "Commands" (built-in, e.g. /compact), "MCP Server Commands", and "ACP Agent Commands" — so same-named commands from different sources show up as distinguishable duplicates. The source category travels on each command's ACP _meta.

Release Notes:

• N/A

[bennetbo]

Not sure if we really need this. I feel like we can get away with typing /compact something something doing nothing (treat it as normal text). Seems like this is what all the other tools are doing too