Register compact command in slash commands palette and registry

[nordicnode]

Summary

• Register the /compact (and alias /summarize) slash command in cli/src/data/slash-commands.ts and cli/src/commands/command-registry.ts.
• Enables user discovery and autocomplete for manual context compaction to reclaim token budget.
• Implements handleCompactCommand in cli/src/commands/compact.ts with early empty-history bailout (Nothing to compact — the conversation is empty.) and mid-turn queue safety (params.addToQueue).
• Adds unit test coverage in cli/src/commands/__tests__/compact-command.test.ts (8 test cases) and updates cli/src/commands/__tests__/command-args.test.ts.

Test plan

[✓] bun test --config=/dev/null --preload ../sdk/test/setup-env.ts src/commands/__tests__/compact-command.test.ts
[✓] bun test --config=/dev/null --preload ../sdk/test/setup-env.ts src/commands/__tests__/router-input.test.ts
[✓] bun test --config=/dev/null --preload ../sdk/test/setup-env.ts src/commands/__tests__/command-args.test.ts
[✓] bun test --config=/dev/null --preload ../sdk/test/setup-env.ts src/data/slash-commands.test.ts
[✓] bun run --cwd cli typecheck
[✓] bun run build:sdk
[✓] bun freebuff/cli/build.ts 0.0.0-ci
[✓] bun cli/scripts/smoke-binary.ts cli/bin/freebuff
[✓] PR hygiene check passed

[codebuff-team]

Nice, focused change that follows the established command pattern (mirrors export-conversation/copy-conversation style handlers) and comes with solid test coverage in compact-command.test.ts covering empty-history bailout, streaming queue, chain-in-progress queue, and alias resolution via findCommand/parseCommandInput. Registration in both slash-commands.ts (palette) and command-registry.ts (dispatch) is consistent and the command-args.test.ts update correctly reflects the new no-args command.

One thing worth double-checking before porting: handleCompactCommand in cli/src/commands/compact.ts doesn't perform any compaction itself — it just calls params.sendMessage({ content: '/compact', agentMode }), i.e. it forwards the literal string /compact as a chat message and relies on the agent runtime to recognize and act on it. If that server-side/agent-side interpretation doesn't already exist, this PR only adds UI discoverability without any actual compaction happening — the message would just be echoed as normal chat input. Please confirm (or link) where /compact is already special-cased in the agent runtime; if it isn't, this needs a companion change there.

Minor nit: the setTimeout(() => params.scrollToLatest(), 0) after sendMessage isn't awaited or covered by tests — worth confirming this pattern matches other commands (e.g. export-conversation.ts) for consistency.

Overall the CLI-side wiring is clean and in scope; flag the backend-behavior assumption for the maintainer doing the port.