fix: Slack message truncation and image attachment handling

[electronicBlacksmith]

What Changed

Message truncation fix:

• When responses exceed Slack's 3000-char block limit, messages are now split into threaded overflow messages with feedback buttons attached to the final chunk
• updateWithFeedback() refactored for multi-chunk thread replies
• postToChannel() accepts optional threadTs for threading

Image attachment processing:

• New slack-files.ts: downloadSlackFiles(), ensureUploadsDir(), cleanupOldUploads()
• PNG/JPEG/GIF/WebP files shared in Slack are downloaded, validated (path traversal + SSRF checks), and passed to the agent as file paths via the Read tool
• Unsupported file types get a friendly skip message
• Old uploads are cleaned up after 24 hours
• Added files:read scope to slack-app-manifest.yaml

Integration:

• Attachment file paths injected into prompt so the agent can read them
• Skipped file reasons surfaced to user in message

Why

Long agent responses were silently truncated at Slack's block character limit, losing content. Image attachments shared in Slack threads were silently dropped with no feedback to the user.

How I Tested

• bun test src/channels/__tests__/slack-files.test.ts - 27 pass (download, validation, cleanup, SSRF, path traversal)
• bun test src/channels/__tests__/slack-formatter.test.ts - 20 pass (split, truncation)
• bun test src/channels/__tests__/slack.test.ts - 38 pass (threading, attachments, feedback)
• bun run lint - clean
• bun run typecheck - clean
• Full suite: 976 pass

Checklist

• Tests pass (bun test)
• Lint passes (bun run lint)
• Typecheck passes (bun run typecheck)
• No secrets or .env files included
• Files stay under 300 lines
• No Cardinal Rule violations
• No default exports or barrel files added