feat(desktop): register Cursor as native ACP runtime + onboarding harness

[Bartok9]

Summary

Register Cursor as a first-class ACP harness and show it on Set up your agent harnesses next to Claude Code and Codex.

Closes #2535. Related: #1683 (prior Cursor PR), #2347 / #2546 (Grok Build companion).

Why this PR exists

Onboarding currently hard-filters visible harnesses to claude + codex only (ONBOARDING_RUNTIME_ORDER). Cursor users cannot Install/sign-in on that page even though Cursor speaks ACP natively:

cursor-agent acp

Critical design note (Grok collision)

Both Cursor and Grok Build install ~/.local/bin/agent. This PR deliberately:

• Uses cursor-agent as the catalog command (not bare agent)
• Only treats path-qualified agent as Cursor when the binary path is under Cursor’s install tree
• Does not process-sweep bare agent (would claim Grok workers)

What changed

• KNOWN_ACP_RUNTIMES: cursor entry (cursor-agent, default args acp, install script, cursor-agent status auth probe, .cursor/skills, ~/.cursor/bin on PATH)
• Readiness + process-sweep binary names (cursor-agent only)
• Onboarding visibility order: claude, codex, cursor
• agentReadiness treats Cursor like other CLI harnesses
• Unit tests for arg normalization, avatar, path disambiguation vs Grok
• buzz-acp README: Cursor path + extension-method follow-up note

Non-claims

• Does not implement auto-allow policy for Cursor blocking extension methods (follow-up).
• Does not implement Grok Build (see feat(desktop): register Grok Build as native ACP runtime + onboarding harness #2546 / feat(agents): add Grok Build as a first-class ACP runtime #2347).

Test plan

• cargo test --lib normalizes_cursor
• cargo test --lib resolves_cursor
• cargo test --lib path_qualified_cursor
• cargo test --lib cursor_runtime
• node --test onboardingRuntimeSelection.test.mjs
• Desktop: harness page shows Cursor Install; after cursor-agent login, Next enables
• Machine with Grok installed: Cursor still resolves via cursor-agent, not Grok’s agent shim

Signed-off-by: Bartok9 danielrpike9@gmail.com

[Bartok9]

Self-review polish landed

Critical finding fixed: Cursor and Grok Build both install a PATH shim named agent (~/.local/bin/agent). On machines with Grok installed first, basename agent is Grok — not Cursor.

Changes in latest push

• Catalog command is cursor-agent only (not bare agent)
• Discovery may still accept a path-qualified agent only when the binary path is clearly under Cursor’s install tree (.local/share/cursor-agent/…)
• Process-sweep no longer lists bare agent (would kill/claim Grok workers)
• Default args / avatar / readiness all key off cursor-agent
• Unit tests cover Grok path non-match + basename rejection

Still out of scope (called out in PR)

• Headless policy for Cursor extension methods (ask_question / create_plan)

[Bartok9]

Polish pass (rebased on upstream/main)

HEAD efb63601 — rebased cleanly onto current main.

Differentiation vs #2546 / competing Cursor PRs

• Binary: prefers cursor-agent only in commands; bare agent accepted only via is_cursor_agent_binary path heuristics (Cursor install tree), never process-swept.
• Why: Grok Build also installs ~/.local/bin/agent — registry must not bind that shim as Cursor.
• Onboarding: still in ONBOARDING_RUNTIME_ORDER as cursor (with claude/codex).
• Docs: crates/buzz-acp/README.md documents cursor-agent + PATH collision warning + headless cursor/ask_question note.

No further code delta this push beyond rebase; collision handling already solid from prior commits on this branch.