Write a per-run fetch log beside the evidence index

[samrusani]

Summary

• A screen writes fetch_log.json at the case-directory root: one row per persisted fetch, with sanitized params and from_cache.
• The evidence index stays a file manifest. No bodies, no keys. --no-write and rerun do not touch the log.
• A symlink at fetch_log.json is refused before any other owned file moves.

Test plan

• Five gates green locally: 766 tests, ruff, mypy, language check
• CI on this PR (pytest matrix, wheel install, pip-audit)

[greptile-apps]

Greptile Summary

The PR adds a per-screen fetch_log.json containing sanitized request metadata and cache status while preserving evidence/index.json as a file manifest.

• Adds fetch-log serialization and guarded persistence beside the casefile and memo.
• Extends overwrite preflight checks to include the new owned path.
• Keeps rerun and no-write flows from creating or modifying the log.
• Documents the new audit artifact and adds CLI, MCP, cache, overwrite, and symlink regression coverage.

Confidence Score: 4/5

The overwrite path should be fixed before merging because a failed fetch-log write can leave an existing case with artifacts from different runs.

The new log is written after overwrite has replaced the evidence and index but before the casefile and memo, while preflight rejects only symlinks, so an invalid non-symlink log path leaves a concretely inconsistent audit pack.

Files Needing Attention: src/coldscreen/casedir.py, src/coldscreen/pipeline.py

Important Files Changed

Filename	Overview
src/coldscreen/casedir.py	Adds fetch-log serialization and owned-path preflight, but an unwritable non-symlink log path can fail after evidence has already been replaced.
src/coldscreen/pipeline.py	Integrates group preflight before overwrite cleanup, though the subsequent multi-file write remains partially destructive if the new log write fails.
tests/test_casedir.py	Covers log contents and symlink refusal but does not cover a directory or other unwritable non-symlink at fetch_log.json.
tests/test_cli.py	Verifies normal screen, no-write, rerun, cache-hit, and evidence-index behavior for the new log.
tests/test_mcp_server.py	Extends MCP persistence and overwrite symlink coverage for fetch_log.json.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Screen gathers NamedRecord entries] --> B{no_write?}
  B -->|Yes| C[Return result without case writes]
  B -->|No| D[Preflight owned paths]
  D --> E{overwrite existing evidence?}
  E -->|Yes| F[Remove old evidence directory]
  E -->|No| G[Write evidence records]
  F --> G
  G --> H[Write evidence index]
  H --> I[Write fetch_log.json]
  I --> J[Write casefile.json]
  J --> K[Write memo.md]

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Prompt To Fix All With AI

### Issue 1
src/coldscreen/casedir.py:193-196
**Partial overwrite corrupts audit pack**

When an overwrite encounters a directory or other unwritable non-symlink at `fetch_log.json`, the symlink-only preflight accepts it and the log write fails after the new evidence and index have been persisted, leaving them alongside the previous `casefile.json` and `memo.md`.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "Write a per-run fetch log beside the evi..." | Re-trigger Greptile}

[greptile-apps]

Partial overwrite corrupts audit pack

When an overwrite encounters a directory or other unwritable non-symlink at fetch_log.json, the symlink-only preflight accepts it and the log write fails after the new evidence and index have been persisted, leaving them alongside the previous casefile.json and memo.md.

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/coldscreen/casedir.py
Line: 193-196

Comment:
**Partial overwrite corrupts audit pack**

When an overwrite encounters a directory or other unwritable non-symlink at `fetch_log.json`, the symlink-only preflight accepts it and the log write fails after the new evidence and index have been persisted, leaving them alongside the previous `casefile.json` and `memo.md`.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.