feat(cli): add experimental cursor-sdk harness

[josemontesdeoca]

Adds cursor-sdk as a third first-party SDK harness in tools/cli, wrapping
@cursor/sdk in local mode behind the existing Harness contract. The
harness ships explicitly as experimental: the default composer-2 model
is tuned for code edits and tool use rather than general instruction-following,
so multi-stage Prose programs may run plan-only. Production runs should still
prefer codex-sdk or claude-sdk.

The change layers cleanly on the existing harness shape — no edits to
canonicalPrompt(), runForwardedProseCommand(), or the two existing SDK
harnesses — and stays within the bounds of the public Harness interface.

What ships

• New harness tools/cli/src/harnesses/cursor-sdk.ts:

  • Dynamic await import("@cursor/sdk") with an injectable factory (mirrors
    the Codex pattern; tests never touch the real SDK).
  • CURSOR_API_KEY for auth; CURSOR_MODEL selects the local model
    (default composer-2).
  • Local agent created with local: { cwd, settingSources: ["project", "user"] }
    so Cursor auto-loads the OpenProse skill.
  • Streams assistant text to stdout; routes tool/status errors to stderr.
  • Exit codes (0 / 1 / 143) and SIGINT/SIGTERM semantics match
    codex-sdk and claude-sdk — abort cancels the active run and disposes
    the agent in finally.

• Registry / typing (tools/cli/src/harnesses/{types,index}.ts):

  • "cursor-sdk" added to HarnessName and HARNESS_NAMES.
  • Cursor SDK types re-exported through types.ts so cursor-sdk.ts is the
    only file importing from @cursor/sdk.
  • createHarness("cursor-sdk") and HarnessSelectionOptions.cursorSdk
    wired through the factory.

• Skill discovery (tools/cli/src/skills/open-prose.ts):

  • New "cursor" SkillAgent value.
  • projectSkillDirectory refactored into projectSkillDirectories so an
    agent can advertise multiple project skill roots; Cursor checks
    .cursor/skills/ first and .agents/skills/ second for cross-client
    interop.
  • User-scope discovery checks ~/.cursor/skills/, ~/.agents/skills/,
    ~/.claude/skills/, and ~/.codex/skills/ per Cursor's documented
    fallback chain.
  • skillAgentsForHarness("cursor-sdk") → ["cursor"]; auto-installer runs
    with --agent cursor via the existing skills@1.5.3 flow.

• Docs and metadata:

  • tools/cli/README.md adds the provider table row, an example invocation,
    a single Harness Details paragraph with a tight Known-limits note, and
    CURSOR_API_KEY / CURSOR_MODEL references.
  • tools/cli/POST_RELEASE_PLAYTEST.md adds Cursor playtest scenarios
    (read-and-explain Prose programs, missing-key path).
  • tools/cli/package.json pins @cursor/sdk: ^1.0.12, refreshes the
    description, and adds the cursor keyword.

• Tests (tools/cli/tests/{harnesses/harnesses,skills/open-prose}.test.ts):

  • Registry: HARNESS_NAMES, resolveHarnessName, and createHarness
    coverage for cursor-sdk.
  • Harness: agent-options shape (apiKey, model.id, local.cwd,
    settingSources), CURSOR_MODEL override, missing-key throws before the
    factory runs, assistant-text → stdout, tool-call/status error → stderr,
    run.wait() error mapping, abort mid-stream returns 143, dispose on
    stream throw.
  • Skill discovery: skillAgentsForHarness mapping, .cursor/skills first
    discovery, four user-scope candidates, ancestor walk across both project
    subdirs, install command targets --agent cursor.