fix(agent): stop unprocessable image retry loops

[tatoalo]

Problem

Once an image the API cannot process enters a cloud session, the same image-processing error can reappear after subsequent tool calls and on later resumes. The agent may continue working, but repeatedly resubmitting the invalid image wastes time and usage and persisted session transcripts can then replay the same invalid content on every resume.

closes #3422

Changes

• Reject unsupported or oversized direct image attachments before they enter the model payload
• Replace invalid images in Read tool output through PostToolUse before the next model request, preventing same-turn retry loops
• Heal persisted session transcripts by replacing invalid top-level and nested tool_result images with a short text notice
• Share the image-size limit, base64 estimator, and neutralization rules across all three paths

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 65e8755.}

[trunk-io]

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[github-actions]

Solid defensive fix for a concrete infinite-retry bug. All three layers (input conversion, PostToolUse hook, session JSONL healer) are purely additive guards — valid images are untouched, only images that would 400 anyway are replaced with text notices. The estimateBase64Bytes extraction from cloud-prompt.ts to @posthog/shared is a clean deduplication with identical logic, and all placements respect the project's import rules. Test coverage is thorough across all new paths including the URL-sourced image (unchanged) edge case.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(agent): guard Read image output befo..." | Re-trigger Greptile}

New commits pushed (delta classified non_trivial_delta) — stamphog approval dismissed; re-review running automatically.

[github-actions]

Solid defensive fix for a concrete infinite-retry bug — valid images pass through unchanged; only images that would 400 anyway are replaced with text notices. The three layers (input conversion, PostToolUse hook, session JSONL healer) are purely additive, import boundaries are respected, and test coverage is thorough across all new paths including the greptile-bot-flagged edge cases (empty data, bare array wrapping) that were fixed in follow-up commits.