[abu-tahir-0]

What this changes

Closes #85.

suite build-images --drop-failed moves a task whose image failed to build into dropped/, but the error only went to stderr. The command now keeps each failed task's error string while building and writes dropped/REASONS.md with a section per slug and the build-log tail, before moving the task files. The terminal output is unchanged.

How it was checked

• make check passes (ruff, mypy --strict, unit tests) — except tests/unit/test_web_vectors.py, which fails identically on a clean main here (t_quantile differs in the last bit); unrelated.
• New behaviour has a test at the cheapest tier that can catch it, including the case that should fail: test_a_dropped_task_leaves_its_reason_in_the_suite runs suite build-images --drop-failed with build_snapshot patched to raise, then asserts the task file moved to dropped/, REASONS.md names the slug, and the error tail is present. No Docker needed.
• make integration / make e2e — not needed; the image build itself is unchanged and the new path is pure file I/O after a build failure.

Anything a reviewer should know

• REASONS.md is rewritten by each --drop-failed run. A later run only lists tasks that run dropped, not the whole history; if you want an append-only log or JSON instead, say so and I will switch the format.
• Errors are fenced as text blocks; a build log that itself contains a four-backtick run could break rendering. I can indent instead if that matters.

[patsypppe]

yes rewriting is fine i dont want to display everything in the history. I will approve the pr