feat(slack-gateway): add workspace install management

[onutc]

Summary

• add authenticated Slack workspace management routes and server-rendered UI in the gateway
• let managers change the selected target or disconnect a workspace through the new backend management APIs
• update the channel install architecture docs with the management surface and ownership details

Testing

• npx -y @simpledoc/simpledoc check
• cd integrations/slack-gateway && go test ./...

[onutc]

Validation update:

• Resolved the merge conflict with main caused by the overlapping Slack install-target backend-routing change and pushed the merge commit.
• Re-ran local validation successfully:
  • cd integrations/slack-gateway && go test ./...
  • npx -y @simpledoc/simpledoc check
• codex review --base main is still blocked in this environment by Codex CLI auth drift (refresh_token_reused / token_expired before review output). I retried it after the merge-resolution commit and it still fails before producing findings.

[onutc]

Final report before merge:

• Implemented authenticated Slack workspace management UI and backend-client plumbing for list, change-target, and disconnect flows.
• Resolved the main conflict caused by the install-target backend-routing change and revalidated locally.
• Validation passed:
  • cd integrations/slack-gateway && go test ./...
  • npx -y @simpledoc/simpledoc check
  • GitHub checks on this PR are green.
• Remaining known blocker is only local Codex CLI auth drift, which prevented codex review --base main from producing findings in this environment.

[gitrank-connector]

👍 GitRank PR Analysis

Score: 20 points

Metric	Value
Component	Other (1× multiplier)
Severity	P2 - Medium (20 base pts)
Final Score	20 × 1 = 20

Eligibility Checks

Check	Status
Issue/Bug Fix	❌
Fix Implementation	✅
PR Documented	✅
Tests	✅
Lines Within Limit	✅

Impact Summary

This PR implements authenticated workspace management routes and UI in the Slack gateway, allowing managers to change installation targets, reconnect, and disconnect workspaces. It includes comprehensive backend API integration, server-rendered HTML templates, and proper authorization via browser principal headers. The implementation follows established patterns and includes detailed architecture documentation.

Analysis Details

Component Classification: This PR adds new workspace management features to the Slack gateway integration, which doesn't fit neatly into predefined categories. It's a feature addition rather than a bug fix or critical system component change.

Severity Justification: This is a Medium (P2) severity contribution. It adds functional capability for managing Slack workspace installations (target selection, reconnection, disconnection) with proper authorization checks, but it's not a critical bug fix or security vulnerability. The feature has clear business value but moderate impact scope.

Eligibility Notes: Tests are required and included: the PR adds 211 lines of test code in gateway_test.go and 122 lines in backend_client_test.go covering the new management endpoints. The PR is well-documented with clear description and related architecture docs. This is a new feature implementation (not a bug fix), so tests are appropriately required and present. All eligibility checks pass.

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