feat: add two-way manifest sync between project and app settings

[mwbrooks]

Changelog

Added slack manifest sync command (and slack sync alias) that detects per-field differences between the local manifest.json and remote app settings, prompts users to resolve conflicts, and writes the merged result to both sides. Gated behind --experiment=manifest-sync.

Summary

This pull request adds two-way manifest sync so developers can resolve differences between their local manifest.json and the remote app settings on api.slack.com.

Today, when manifest.source=local, the local manifest silently overwrites the API on next install — discarding changes made on the web. This feature replaces the binary "Overwrite?" prompt with a per-field resolution flow when divergence is detected.

Key additions:

• internal/pkg/manifest/ — flatten, diff, merge, display, writeback, and sync orchestrator
• cmd/manifest/sync.go — standalone slack manifest sync command
• slack sync alias in cmd/root.go
• manifest-sync experiment flag for gated rollout
• Integration into shouldUpdateManifest() in the install flow

Preview

📚 App Manifest
   Found 2 difference(s) between project and app settings

  display_information.description
    Project:      "A cool app for teams"
    App settings: "A cool app for collaboration"

  functions.new_func (only in project)
    Value: {"title":"New Function","description":"Greets"}

? How would you like to resolve these differences?
  > Use all project values
    Use all app settings values
    Choose for each difference

  Syncing manifest...
  ✓ Updated app settings
  ✓ Updated manifest.json

📚 App Manifest
   Finished manifest sync for "My App"

Testing

1. Build the CLI: make build
2. In a project with manifest.json, install an app: ./bin/slack install -e manifest-sync
3. No differences: Run ./bin/slack manifest sync -e manifest-sync — should report "in sync"
4. Remote change: Edit the manifest on https://app.slack.com/app-settings, then run ./bin/slack manifest sync -e manifest-sync — should show the difference, choose "Use all app settings values", verify manifest.json updated
5. Local change: Edit manifest.json, run ./bin/slack manifest sync -e manifest-sync — choose "Use all project values", verify app settings updated
6. Both sides changed: Edit manifest.json and app settings (different fields), run sync, choose "Choose for each difference" — verify per-field resolution works and both sides reflect the merged result
7. Key order preserved: After sync, git diff manifest.json should only show value changes, not key reordering
8. Triggered during install: Make a remote change, run ./bin/slack install -e manifest-sync — sync flow triggers instead of old "Overwrite?" prompt
9. Force flag: Make a remote change, run ./bin/slack install -e manifest-sync --force — old behavior, local overwrites remote
10. Without experiment: Run ./bin/slack install with a remote change — old "Overwrite?" prompt (unchanged behavior)

Notes

• The set-manifest hook for code-generated manifests (e.g., manifest.ts) is not included in this initial implementation. For those projects, the merged manifest is pushed to the API only, with a warning that the local project was not updated.
• Uses a two-way (local vs remote) comparison model rather than three-way merge. This avoids stale cached base issues in multi-developer scenarios.
• The --force-remote flag (remote overwrites local) is a follow-up item.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 44.01806% with 248 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.07%. Comparing base (6dc5f98) to head (6f49023).

Files with missing lines	Patch %	Lines
internal/pkg/manifest/display.go	0.00%	106 Missing ⚠️
internal/pkg/manifest/sync.go	0.00%	70 Missing ⚠️
internal/pkg/manifest/writeback.go	57.69%	19 Missing and 14 partials ⚠️
cmd/manifest/sync.go	65.51%	10 Missing ⚠️
internal/pkg/manifest/merge.go	87.34%	5 Missing and 5 partials ⚠️
internal/pkg/manifest/diff.go	82.97%	4 Missing and 4 partials ⚠️
internal/pkg/apps/install.go	0.00%	4 Missing and 3 partials ⚠️
internal/pkg/manifest/flatten.go	84.61%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #543      +/-   ##
==========================================
- Coverage   71.66%   71.07%   -0.60%     
==========================================
  Files         226      233       +7     
  Lines       19176    19617     +441     
==========================================
+ Hits        13743    13942     +199     
- Misses       4222     4440     +218     
- Partials     1211     1235      +24     

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