Fix Windows 32KB command-line limit for agent prompts

[wesm]

Summary

• Switch copilot, cursor, droid, and opencode agents from CLI arguments to stdin piping for prompt delivery, avoiding the Windows CreateProcess 32,766-character limit
• Switch gh pr comment from --body to --body-file - with stdin piping for the same reason
• Add cmd.WaitDelay = 5s to all pipe-based agents (claude, codex, cursor, gemini) so context cancellation reliably terminates pipe reads when child processes outlive the parent
• Add --large-prompt flag to check-agents for validating the fix with a 33KB+ prompt

Test plan

• go test ./... passes
• roborev check-agents — all agents pass
• roborev check-agents --large-prompt — agents that support large prompts pass

Closes #266

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review

Verdict: Changes are directionally good, but there are 3 Medium-severity issues to address before merge.

Medium

1. No compatibility fallback for new stdin prompt mode

   • Refs: internal/agent/copilot.go:79, internal/agent/droid.go:98, internal/agent/opencode.go:118, internal/agent/cursor.go:107
   • Issue: These agents now send prompts via stdin without a capability/version fallback. Older or variant CLIs that require positional/flag prompt input may fail or become interactive.
   • Recommendation: Add per-agent capability detection (or version gating) and fallback to prior argument-based prompt mode when stdin prompt mode is unsupported.

2. ClaudeAgent appears not migrated to stdin prompt handling

   • Refs: internal/agent/claude.go
   • Issue: ClaudeAgent is inconsistent with other agents and appears to still pass prompt content via command arguments, keeping process-list exposure and argument-length risk.
   • Recommendation: If supported by the Claude CLI, pipe prompt via cmd.Stdin and remove prompt text from command arguments.

3. Some stdin-piped agents lack WaitDelay

   • Refs: internal/agent/copilot.go:85, internal/agent/droid.go:104, internal/agent/opencode.go:131
   • Issue: These subprocesses now use pipes but do not set cmd.WaitDelay, increasing hang risk if pipe closure/drain behavior is abnormal.
   • Recommendation: Set cmd.WaitDelay consistently for these agents, matching the safer pattern used elsewhere.

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Synthesized from 4 reviews (agents: codex, gemini | types: security, default)

[roborev-ci]

roborev: Combined Review

Verdict: Changes are mostly positive, but there are 2 Medium-severity functional regressions to address before merge.

Medium

1. Potential Copilot prompt delivery regression

   • File: internal/agent/copilot.go:79
   • Issue: Prompt transport was changed to stdin only, and --prompt was removed. If Copilot CLI still requires --prompt for non-interactive mode, prompts may be ignored or the process may hang waiting for interaction.
   • Suggested fix: Keep an explicit --prompt path, or add CLI capability detection/fallback between flag-based and stdin-based prompt input.

2. Potential OpenCode invocation semantic mismatch

   • File: internal/agent/opencode.go:118
   • Issue: Code now pipes prompt via stdin and no longer passes positional message args, while comments/documented usage indicate opencode run [message..]. If message args (or explicit stdin marker) are required, behavior may regress.
   • Suggested fix: Use the CLI’s documented explicit stdin form (if supported), or retain positional prompt passing with guarded fallback for large prompts.

Notes

• No Critical or High findings were reported.
• Security-focused reviews reported no security vulnerabilities in this diff.

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Synthesized from 4 reviews (agents: gemini, codex | types: security, default)