feat: bundle a slim goose agent (goose-acp) as a desktop sidecar

[michaelneale]

Bundles a slim goose acp binary as a desktop sidecar so Buzz ships a working goose agent out of the box — no goose install required.

How it's made small

• New goose-acp/ crate (workspace-excluded, own lockfile): a 14-line main calling goose::acp::server::run() on the goose library crate with default-features = false, features = ["rustls-tls"] — drops code-mode (V8 JS runtime), local-inference (llama.cpp), keyring, AWS providers, telemetry, and the goose CLI/TUI entirely
• Size-optimized profile (opt-level=z, fat LTO, panic=abort, strip)
• rmcp pinned at 1.7.0 in the crate's own lockfile (goose doesn't compile against 1.8); workspace Cargo.lock is untouched by goose's dependency graph

Net impact

Binary (macOS arm64)	32 MB (vs ~240 MB stock goose install)
Download delta	+9 MB compressed (+12% on the 72 MB DMG)
Future	drops to ~14 MB raw / ~+5 MB download when goose's upstream analyze/tiktoken feature gates land (rev bump only)

Wiring (~60 LOC outside the new crate)

• buzz-acp: goose-acp identity takes no acp subcommand (it is the ACP server), + tests
• Desktop: goose runtime discovery accepts goose-acp as fallback — user-installed goose still wins
• Packaging: externalBin entry, sidecar stubs (justfile + CI), bundle-sidecars.sh stages it when built (with chmod +x — cp onto a stub keeps the stub's mode), release workflow builds it for all targets

Verified

• ACP initialize handshake against the built binary
• Full E2E: local relay → buzz-acp → bundled goose-acp → Anthropic → reply posted in channel
• Desktop Tauri crate builds against the staged sidecar; just dev runs

[michaelneale]

CI note: the one red check, Desktop Smoke E2E (2) (human-edit-agent-content.spec.ts), is pre-existing on main — the same 6 tests fail identically on a clean origin/main checkout locally, and the two CI runs on main before 8174f2b0 show the same Desktop Smoke E2E (2) failure. Unrelated to this change (which touches no desktop UI code paths). Everything else is green, including Desktop Build (macOS), which stages the new goose-acp sidecar stub.

[tlongwell-block]

@michaelneale can you look at the way we do auto-installs for the ACP adapters for claude code and codex at boostrap time as part of user onboarding and refactor this to where this slim goose is a one click install on that screen?

[michaelneale]

@tlongwell-block I could - but partly defeats the purpose, this bundles the small binary in (a few MB extra) so no download opt-in needed (which means less potential security hassles) - but could do that too (ie pull down a lean agent). the idea here is to show how small a combination batteries included could be (ie only a small addition to the net size of buzz) - but it isn't clear from the description I guess. More if they choose to use it would be nothing extra.

[tlongwell-block]

We already bundle a full agent, @michaelneale

I think at some point we might want to cut over to goose, but right now for 5-10MB total the buzz-agent is working really well for us

[michaelneale]

@tlongwell-block tnx, no worries