fix: include review context when custom fix instructions are provided

[mariusvniekerk]

Summary

• When a user provides custom instructions via Shift+F in the TUI fix modal, the custom prompt was completely replacing the review context instead of being appended to it
• The fix agent received only the user's short instructions with no knowledge of what the review actually found, causing it to search aimlessly and eventually give up
• Custom instructions are now combined with the review findings via buildFixPromptWithInstructions, so the agent sees both the original review output and the user's additional guidance

Closes #387

Test plan

• New test: "custom prompt includes review context" verifies stored prompt contains both review output and custom instructions
• All 3521 existing tests pass
• Manual: Shift+F on a review, enter custom instructions, verify agent log shows review findings in the prompt

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review (85f3789)

Summary Verdict: The code changes introduce a potential behavioral regression regarding how conflicting job inputs are handled, while no security vulnerabilities were
found.

Medium

Regression/Behavior Change

• File: internal/daemon/server.go (in handleFixJob)
• Details: The condition changed from
  if fixPrompt == "" && req.StaleJobID > 0 to if req.StaleJobID > 0. This means if both StaleJobID and Prompt are sent, Prompt is now silently ignored.
• Suggested Fix: Make this explicit in API behavior by
  either:
  1. Rejecting StaleJobID + Prompt with a 400 and a clear error, or
  2. Incorporating Prompt into the rebase prompt path.

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

[roborev-ci]

roborev: Combined Review (522c440)

Verdict: All agents reviewed the changes and found no issues of medium, high, or critical severity.

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