Add first-class rho agent support

[bpmooch]

Summary

Add first-class rho support to dn so users can run dn --agent rho (and related env toggles) alongside OpenCode, Cursor, Claude Code, and Codex.

This depends on the harness CLI already implemented in rho (rho dn subcommand).

Goal

• dn prep, dn loop, dn kickstart, dn meld, dn fixup, and scoring paths that invoke an agent harness can select rho the same way they select other backends.
• Generated GitHub Actions workflows from dn init build can record rho as the agent choice where applicable.

Key changes

Type and registry

In sdk/github/agentHarness.ts:

• Extend AgentHarness with "rho".
• Append "rho" to AGENT_HARNESSES.
• In getRunAgent, return a new runRhoAgent when harness is "rho".
• In resolveAgentHarnessFromFlagsAndEnv:
  • Accept explicit --agent rho.
  • Support RHO_ENABLED=1 as an env toggle (mutually exclusive with CURSOR_ENABLED, CLAUDE_ENABLED, CODEX_ENABLED, OPENCODE_ENABLED).

Runner implementation

Add sdk/github/rhoAgent.ts (aligned with cursorAgent.ts, claudeAgent.ts):

• Signature: match RunAgentFn: (phase: "plan" | "implement", combinedPromptPath: string, workspaceRoot: string, _useReadonlyConfig?: boolean) => Promise<{ code: number; stdout: string; stderr: string }>

• Pre-flight: verify rho is on PATH; throw clear install message if missing.

• Path handling: stat combinedPromptPath, require a file, realPath to absolute.

• Spawn: Deno.Command with:

  • cwd: workspaceRoot
  • argv: invoke rho dn --combined-prompt <absolutePath> --phase <phase>
  • stdin: "null"
  • stdout/stderr: "piped", collect into strings

• Timeout: read RHO_TIMEOUT_MS, then OPENCODE_TIMEOUT_MS, then default 600000; race subprocess with timer.

• Logging: emit [dn] Running rho … prefix before spawn.

CLI wiring

• Global --agent rho must parse and flow into orchestrator config the same as --agent cursor.

dn init build

• Extend agent enum so interactive and non-interactive flows can choose rho.
• Generated workflow YAML: include instructions for required secrets.

Documentation

• README: agent table row for rho (install rho, put on PATH, set provider API keys).
• docs/subcommands.md: examples for dn --agent rho prep, loop, kickstart, meld, fixup.
• docs/output-and-environment.md: harness table row for rho; document RHO_TIMEOUT_MS, RHO_ENABLED.

Acceptance criteria

• With rho on PATH and required credentials: dn --agent rho prep <issue> completes plan phase with exit 0.
• dn --agent rho loop --plan-file plans/…plan.md completes implement phase with exit 0.
• dn init build --agent rho generates a workflow documenting rho and required secrets.
• Docs list rho next to other agents.

Boundaries

• dn does not bundle the rho binary; users install rho separately.
• Document a minimum rho version once the harness subcommand is stable.

Dependencies

• 008-rho-dn-harness-cli.md - harness CLI already implemented in rho binary as rho dn subcommand