refactor(buzz-agent): swap the agent loop onto the goose library

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "crates/buzz-audit",
     "crates/buzz-acp",
     "crates/buzz-agent",
+    "crates/buzz-model-catalog",
     "crates/sprig",
     "crates/buzz-test-client",
     "crates/buzz-ws-client",

crates/buzz-agent/Cargo.toml

@@ -25,6 +25,22 @@ name = "fake-mcp"
 path = "tests/bin/fake_mcp.rs"
 
 [dependencies]
+# For the `buzz-agent auth <provider>` subcommand only. Provider auth for the
+# agent loop itself is goose's; this is the interactive PKCE login that seeds
+# the token cache that goose (and the desktop model picker) then read.
+buzz-model-catalog = { path = "../buzz-model-catalog" }
+goose = { git = "https://github.com/aaif-goose/goose", rev = "305849b71709b95b86ed9f11bd3bc939899c0aab", default-features = false, features = ["rustls-tls"] }
+goose-provider-types = { git = "https://github.com/aaif-goose/goose", rev = "305849b71709b95b86ed9f11bd3bc939899c0aab" }
+# Resolution guard: goose pins icu_locale "=2.1.1" (needs icu_collections
+# ~2.1.1) while url 2.5.x -> idna -> idna_adapter 1.2.2 pulls icu_normalizer
+# 2.2.0 (needs icu_collections ~2.2.0). Only one 2.x icu_collections can be
+# selected. 1.2.0 is the last idna_adapter on ICU4X 1.x, which keeps IDNA off
+# the icu_collections 2.x line entirely and lets both resolve.
+idna_adapter = "=1.2.0"
+tokio-util = { version = "0.7", features = ["rt"] }
+futures = "0.3"
+anyhow = "1"
+
 tokio = { workspace = true, features = ["rt-multi-thread", "macros", "io-std", "io-util", "sync", "process", "time", "net"] }
 serde = { workspace = true }
 serde_json = { workspace = true }

crates/buzz-agent/HANDTEST.md

@@ -0,0 +1,96 @@
+# Hand-testing `buzz-agent`
+
+This is a **swap**, not a new thing. `buzz-agent` keeps its name, its binary,
+its ACP contract and its place in the workspace — only the internals changed
+from a hand-written agent loop to the `goose` crate used as a Rust library.
+
+So there is nothing special to run. The normal flow *is* the test:
+
+```bash
+just dev            # the whole app
+# or
+just relay          # terminal 1
+just goose          # terminal 2 — agent against that relay
+```
+
+Both already build and use the swapped `buzz-agent`. If you see a difference,
+that is the bug.
+
+## What to look at
+
+Ordered by risk. The automated suite (41 tests) covers each of these at the
+stdio layer; this list is the part only a human can judge.
+
+### 1. Persona actually arrives
+
+```
+@fizz who are you?
+```
+
+Should answer *as Fizz* and know the `buzz` CLI exists. A generic goose answer
+means the system prompt was dropped — the exact failure that makes plain-ACP
+embedding impossible, and the reason this uses the library API instead.
+
+### 2. `_Stop` veto
+
+```
+@fizz make a todo list with 3 items, then stop immediately without doing them
+```
+
+Must refuse to stop while items are open. Capped at 3 vetoes, so it ends
+eventually regardless. Look for `_Stop hook vetoed end of turn` in the log.
+
+### 3. Streaming feel
+
+The old loop emitted one chunk per round; goose streams token-by-token. Tests
+prove the relay is not write-amplified (chunks are coalesced by identity key,
+flushed at 500ms, paced at 167ms/90-per-minute), but **only a human can say
+whether it feels better or worse in the desktop app.** This is the most likely
+source of "something is off".
+
+### 4. Cancel mid-tool
+
+Ask for something long (`count slowly to 100 with a shell sleep`), then stop.
+The turn should end promptly with **no tool call left spinning**. Cancellation
+is a cooperative drain with a 5s budget — dropping the stream instead would
+leave the MCP child running and the spinner stuck forever. That bug existed and
+is fixed; this is the visual confirmation.
+
+### 5. Steering
+
+Send a second message while the agent is working. It should be absorbed into
+the running turn, not cancel-and-restart it.
+
+### 6. Model picker
+
+Should list models, not just the current one. An absent catalog is degraded UX,
+never a session failure.
+
+### 7. Tool hygiene — known deviation
+
+The model can now *see* `_Stop` and `_PostCompact`: goose's allowlist gates
+advertising and dispatch through the same cache, so hiding them would also make
+them undispatchable and break the veto. A system-prompt extension tells the
+model to leave them alone. Watch for it calling `_Stop` itself — if that
+happens the guidance needs strengthening.
+
+## Comparing against the old behaviour
+
+The old loop is gone, so A/B means checking out `main` in a second worktree and
+running that relay side by side.
+
+## Known gaps
+
+* **Never run against a real provider.** All automated coverage uses a fake SSE
+  server. Databricks *chat* now goes through goose entirely and is unexercised
+  — though Databricks *model discovery* still uses our own code, moved to
+  `buzz-model-catalog` (the desktop cannot link goose: native `sqlite3`
+  collision with its own rusqlite).
+* **Relay-mesh MoA is restored but only tested against a fake router.** With
+  `mesh-llm` running, a relay-mesh agent on `auto` should switch to the virtual
+  `mesh` model within ~2 turns once ≥2 models are live, and fall back to `auto`
+  within 30s of the mesh shrinking. Grep the agent log for
+  `relay-mesh auto:`.
+* Binary is roughly +22.7 MiB raw / +6.0 MiB gzip.
+* Nothing changed in packaging or the harness catalog — same sidecar name, so
+  nothing needed to.