[Fix] Visual-proof auto-posts to Slack when experimental auto proof is off

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

Related issue

N/A — internal fix identified while investigating unexpected Slack visual-proof auto-posts with the experimental flag off.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

What changed

Visual-proof artifact auto-post to Slack/chat is now actually gated by SlackProofAutoPost / ROOMOTE_SLACK_PROOF_AUTO_POST.

Previously, the experimental flag only set the runtime env used for prompt wording, while the worker still auto-posted the first visual-proof upload whenever chat reply context existed. The platform path now short-circuits unless the flag env is enabled.

Slack agent instructions and manage_artifacts MCP descriptions also match that gate:

• flag on: keep “built-in proof already posted” guidance
• flag off: tell the agent to share proof via send_chat_reply / imageArtifactIds instead of claiming automatic posting

How it was tested

• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/worker exec vitest run src/mcp/roomote-mcp-server/__tests__/upload.test.ts src/mcp/roomote-mcp-server/__tests__/tool-descriptions.test.ts — 36/36 passed
• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/cloud-agents exec vitest run src/server/workflows/__tests__/slackAppMention.test.ts -t 'documents' — passed
• Pre-push lint / typecheck / knip hooks passed on commit

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

[roomote-roomote]

No code issues found. See task

Verified the gate is wired end to end: the worker sets ROOMOTE_SLACK_PROOF_AUTO_POST=true from the SlackProofAutoPost deployment flag, replyToChatWithVisualProof now short-circuits on isVisualProofAutoPostEnabled() before the only auto-post call site (handleUpload), and the prompt/MCP wording is driven by the same deployment flag so the agent guidance matches actual behavior. The added tests cover both the flag-off no-post path and the flag-on paths.

Note (not blocking): the 3 failing tests in slackAppMention.test.ts are pre-existing on develop (deployment PR-action wording assertions unrelated to this change), and this PR adds one new passing test.

_{Reviewed 8f154d1}