Follow-up: complete Slack attachment contract parity + hardening after PR #601

[Aaronontheweb]

Context

PR #601 fixed the primary live-ingress bug (non-image Slack attachments being silently dropped), but we intentionally deferred several follow-ups that are still worth landing to close correctness and hardening gaps.

Why this issue

Live Slack ingress is now attachment-aware, but historical thread hydration and a few robustness items remain inconsistent with the intended contract.

Follow-up tasks

• Backfill parity (completeness): update src/Netclaw.Channels.Slack/SlackThreadHistoryFetcher.cs so historical thread hydration no longer hard-filters to image/* only.
  • Current filter is around SlackThreadHistoryFetcher.cs:146.
  • Ensure historical PDFs/docs are handled consistently with live ingress behavior.
• User-facing error hygiene: in src/Netclaw.Channels.Slack/SlackThreadBindingActor.cs, avoid exposing raw ex.Message in channel replies on download/scan/write failures.
  • Keep exception details in logs; return stable user-safe messages in Slack.
• Attachment-line safety: harden [attachment] line construction to normalize control characters/newlines in metadata before emitting text.
  • Current escaping (EscapeQuoted) handles quotes/backslashes only.
• Regression tests: add tests for
  • historical non-image backfill behavior,
  • control-char/newline sanitization for attachment metadata,
  • sanitized user-facing failure text.
• Ops skill sync: update feeds/skills/.system/files/netclaw-operations/SKILL.md guidance to match inbox + attachment-line behavior.

Acceptance criteria

• Historical backfill no longer silently omits non-image attachments.
• No raw exception internals are sent to Slack users from attachment ingest failures.
• Attachment announcement lines remain single-line and parse-safe even with hostile filename metadata.
• New tests cover the above cases and pass in CI.

Related

• PR: feat(channels): audience-gated attachment ingress contract + Slack PDF support #601