feat(gateway): ingest inbound Slack text/log attachments (#1718)

[kgabryje]

Summary

Extends the inbound Slack attachment ingestion pipeline (#1847) from images-only to text-like files agents use as context: logs, plain text, CSV, JSON, and markdown.

• Filter generalized: isIngestableImageFile → isIngestableFile accepts image/* plus the text-like subset (text/*, application/json) of the existing ALLOWED_UPLOAD_MIME_TYPES upload-route allowlist — no new MIME list is introduced; the security boundary stays defined in one place.
• Response content-type validation broadened the same way: the download path now accepts allowlisted text types alongside images, and keeps rejecting everything else (text/html login pages, image/svg+xml, scripts, and allowlisted-but-out-of-scope types like application/pdf).
• Rename: ingestInboundImageAttachments → ingestInboundAttachments. All hardening is unchanged: slack.com host allowlist with per-hop redirect revalidation, streaming byte-cap on actual received bytes, per-file size and per-message count ceilings, file.id-prefixed filenames, graceful degradation (failures append a note, never throw).
• Reuses the existing ingest_files flag and files:read scope — no new flag or scope. UI labels/tooltips and the MCP tool description updated from "images" to "images and text files".
• Gateway service call site unchanged in logic — the filter inside the ingest function now decides image vs. text vs. skip.

Out of scope (deferred to #1719): PDFs, office documents, OCR, media.

Tests

• Text attachment (text/csv) downloaded, stored, and path folded into the prompt
• isIngestableFile accepts text/plain, text/csv, text/markdown, application/json; rejects application/x-sh, application/xml, image/svg+xml, text/html, and allowlisted-but-out-of-scope application/pdf and application/zip
• Oversized text stream aborted by the streaming byte-cap
• Allowlisted-but-out-of-scope response content-type (application/pdf) rejected
• ingest_files off → no download (existing coverage, unchanged)

Full daemon suite: 1643 passed. pnpm check green.

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