fix: skip file download errors in search instead of aborting

[shhac]

Problem

When a Slack file attachment returns a 401 (no read permission) or 404 (deleted), downloadSlackFile throws.

• The message-read path (message get, message list) handles this gracefully — downloadMessageFiles wraps each download in a try/catch, logs a warning, and continues.
• The search paths (search-messages, search-files) call downloadSlackFile directly with no error handling, so a single inaccessible attachment kills the entire search operation.

This commonly happens with:

• Deleted attachments
• Files forwarded from a workspace/channel the caller doesn't have access to

Fix

Wrap each downloadSlackFile call in search-messages and search-files with try { … } catch { continue }

• Matches the existing bare-catch convention used throughout src/slack/ for tolerable errors (e.g. channels.ts, messages.ts, search-messages.ts message enrichment).
• Failed downloads are silently skipped; remaining results still return normally.

Notes

• Canvas fetching (canvas.ts) is intentionally left unchanged — there, the file download IS the primary operation, so a failure should still surface as an error.
• IMO it might be a nicer fix to offer a sibling to downloadSlackFile, downloadOptionalSlackFile which handles the try-catch on that side, but this doesn't look like the pattern in this repo

[AmethystLiang]

This is the Codex review. Once addressed, will merge the PR for you.

High

• src/slack/search-messages.ts:268 causes search messages --content-type file|image|snippet to drop valid matches whenever the attachment download fails. The new catch { continue; } means downloadedPaths[f.id] is never set, and later src/slack/search-messages.ts:222 derives hasFiles from compact.files, which only includes successfully downloaded files. So a message that does contain an image/snippet/file is now treated as file-less and filtered out. That changes search semantics, not just error handling.

Medium

• src/slack/search-files.ts:49, src/slack/search-files.ts:141, and src/slack/search-messages.ts:268 swallow every exception from src/slack/files.ts:34, not just the intended 401/404 cases. That also hides network failures, malformed responses, temp-dir/write failures, and auth regressions as silent partial success. The existing message-read path logs a warning instead of failing hard; it does not suppress the failure completely (src/cli/message-file-downloads.ts:98). This PR should at least narrow the skipped cases or log them.

No tests cover these search download/error paths, so the regression risk here is higher than usual.

[shhac]

Addressed both issues (note: this increases scope):

High — failed downloads dropping messages from content-type–filtered search results

The root cause was that downloadedPaths had no entry for failed files, so toCompactMessage excluded them from compact.files, making passesContentTypeFilter treat the message as file-less. Fix: downloadedPaths now stores DownloadResult objects (success or failure). Failed downloads produce a compact file entry with error instead of path, preserving the file metadata (mimetype, mode) so content-type filtering still works.

Medium — swallowing all exceptions silently

Introduced SlackDownloadError to distinguish network/HTTP errors from disk/write errors. tryDownloadSlackFile catches only SlackDownloadError and returns a structured failure result; disk errors propagate. All call sites (including canvas downloads) now log warnings on failure. The boundary: we defend against unavailability from the remote, not a broken local system.

Added 18 tests covering the download wrapper, toCompactMessage with error entries, and the content-type filter integration.

Example search output with a failed download

{
  "messages": [
    {
      "ts": "1710000000.000001",
      "author": { "user_id": "U12345" },
      "content": "Here's the screenshot",
      "files": [
        {
          "mimetype": "image/png",
          "path": "/tmp/agent-slack-downloads/F0001.png"
        }
      ]
    },
    {
      "ts": "1710000000.000002",
      "author": { "user_id": "U67890" },
      "content": "Check this attachment",
      "files": [
        {
          "mimetype": "image/jpeg",
          "error": "Failed to download file (401)"
        }
      ]
    }
  ]
}

[AmethystLiang]

Reviewed latest branch state locally after the follow-up fixes. Full test suite and typecheck pass, and the download-error contract/docs are now aligned.