feat(acp): add Letta as a tier-2 preset harness

[sarahwooders]

Adds Letta to the runtime catalog. Letta agents are stateful — memory persists across sessions and channels — so a Buzz agent backed by Letta accumulates context in the room it lives in rather than starting cold each turn. It speaks ACP through the letta-acp adapter (npm install -g @letta-ai/letta-acp).

Duplicates: none found for Letta. The closest open PRs are the other preset additions — #3450 (Cybara), #3287 (Kiro), #3225 (Devin) — and #3225 in particular proposes the same discovery/presets.rs extraction this PR needed (see below); happy to rebase onto whichever lands first.

Changes

• PRESET_HARNESSES entry — letta / letta-acp, no args. preset_harness_ids() derives the reserved-id set from this slice, so nothing else needs updating to keep custom harnesses from shadowing it. (The BYOH contributor guide's step 3, "add the preset id to BUILTIN_IDS", is stale relative to that — happy to fix it here if you'd like.)
• default_agent_args — letta-acp is a zero-arg runtime, like claude-agent-acp and codex-acp. Without this, the legacy Goose-shaped acp default reaches it as a positional.
• Preset data moved to discovery/presets.rs — second commit, pure code motion. discovery.rs is 1860 lines, over the 1000-line desktop file-size limit, so the ratchet in pnpm check forbids any growth and a new preset entry cannot land without this. Moving the tier-2 block drops it to ~1716 and gives presets a small file to grow in; the presetLogos guard now parses the new path.
• Logo — inline RUNTIME_MARKS entry rather than a bitmap, so it adapts to dark/light. The mark comes from Letta's own Apache-2.0 letta-evals repo (leaderboard_site/src/letta-logo.svg, commit 2daaf0c); provenance recorded in CREDITS.md, and it already ships as currentColor paths.
• Catalog copy — one neutral sentence, matching the existing content policy.
• buzz-acp README — a "Running with Letta" section plus a backend table. Letta can run agents locally, on a self-hosted app server, or in Letta Cloud, and the choice decides where tools execute. The agent answers Buzz by shelling out to the buzz CLI, so the CLI has to be wherever that is: local and cloud-oauth inherit the harness environment; remote needs the CLI on the app server; plain cloud runs tools in Letta's sandbox and is not usable from Buzz.

Testing

• cargo test -p buzz-acp --lib — 628 passed.
• cargo test --lib managed_agents in desktop/src-tauri — 896 passed.
• pnpm check (lint + format + file-size ratchet), pnpm test (3770 passed, including the presetLogos guard for the new id), and pnpm typecheck in desktop — all clean.
• cargo fmt --all -- --check and the Tauri equivalent; cargo clippy --all-targets -- -D warnings for buzz-acp and buzz-desktop.
• End to end: buzz-acp models --agent-command letta-acp against a locally built buzz-acp reports Agent: letta-acp v0.1.4 and completes initialize + session/new, on Letta's local, remote, and cloud-oauth backends.

Screenshots of the runtime catalog and the harnesses panel to follow.

Companion PR on the adapter side: letta-ai/letta-acp#28 teaches letta-acp to connect the MCP servers Buzz passes in session/new, honor a client-supplied systemPrompt, and report agentInfo (which is what turns Agent: unknown vunknown into a real name in the probe above). This PR does not depend on it — the CLI path works with the published 0.1.4.