[Fix] Derive SLACK_AUTH_URI from ROOMOTE_APP_URL when unset

[mrubens]

Problem

When SLACK_AUTH_URI is unset, the Slack account-linking onboarding DM silently fails. The env schema defaults it to an empty string, so the "Link accounts" button URL becomes ?state=<token> — Slack rejects the entire chat.postMessage with invalid_blocks ("invalid url"), and unlinked users who DM the bot get no response at all.

This hit the roomote-nightly Railway deployment on 2026-07-10: none of the fleet deployments set SLACK_AUTH_URI (the ops provision script only sets ROOMOTE_APP_URL).

Fix

When SLACK_AUTH_URI is unset or empty, derive it from ROOMOTE_APP_URL as ${ROOMOTE_APP_URL}/api/slack/auth — the route the web app already serves for completing the link (apps/web/src/app/api/slack/auth/route.ts). An explicitly set SLACK_AUTH_URI still wins.

The fallback lives in buildRoomoteRuntimeEnv (packages/env) rather than at the button-building site, so every service that constructs env — including rehydrateEnv and the web bootstrap — gets it, matching the existing derivation pattern for SLACK_API_TIMEOUT_MS and DOCKER_WORKER_IMAGE.

Changes

• packages/env/src/index.ts — derive SLACK_AUTH_URI from ROOMOTE_APP_URL when unset/empty (trailing slashes trimmed)
• packages/env/src/__tests__/index.test.ts — tests for the fallback (unset, empty, trailing slash) and explicit-override precedence
• apps/docs/environment-variables.mdx — document the default

Validation

• pnpm --filter @roomote/env exec vitest run src/__tests__/index.test.ts — 53 passed
• pnpm --filter @roomote/slack exec vitest run src/__tests__/block-kit.test.ts — 21 passed
• pnpm lint and pnpm check-types — clean across all workspaces

No fleet config change needed: deployments already set ROOMOTE_APP_URL, so the next deploy with this commit fixes the silent DM failure.

[roomote-roomote]

No code issues found. See task

Verified the fallback lives in buildRoomoteRuntimeEnv, so it flows through createRoomoteEnv, rehydrateEnv, and the shared Env proxy for every service. The derived ${ROOMOTE_APP_URL}/api/slack/auth matches the route that reads ?state= (apps/web/src/app/api/slack/auth/route.ts) and is consistent with ecosystem.config.js. Precedence (explicit wins), empty/unset handling, and trailing-slash trimming are all correct, and docs/tests are in sync.

[mrubens]

@openmote should we do the same for SLACK_REDIRECT_URI?

[roomote-roomote-v1]

I saw the mention, but I could not process it right now because the GitHub follow-up context was incomplete. Please try again in a moment.