fix: preserve group name when promoting to local folder group

[PureWeen]

Problem

PromoteOrCreateLocalFolderGroup and ReconcileOrganization's external worktree migration both unconditionally renamed groups to Path.GetFileName(worktreePath) during promotion. This destroyed user-customized group names.

Repro: User has a group named "maui" for dotnet/maui sessions. They add ~/Projects/maui2 as an existing folder. The promote code finds the "maui" group (RepoId=dotnet-maui), sets LocalPath = ~/Projects/maui2, and renames it to "maui2" (Path.GetFileName("~/Projects/maui2")). The user's 17-session "maui" group is now called "maui2" and converted to a local folder group. A new orphan "maui" group is auto-created with only 1 session.

Root Cause

Two locations in CopilotService.Organization.cs overwrite the group name during promotion:

1. PromoteOrCreateLocalFolderGroup (line 1536): candidate.Name = Path.GetFileName(normalized);
2. ReconcileOrganization migration (line 744): groupToPromote.Name = Path.GetFileName(normalizedExtPath);

Fix

Remove the Name override in both locations. Promotion should only set LocalPath to convert the group to a local folder group — the user's chosen group name must be preserved.

Testing

• 2 new regression tests: PreservesUserGroupName_WhenFolderNameDiffers and PreservesGroupName_WhenPromoting
• All 8 promotion-related tests pass
• Full suite: 3477 tests pass, 0 failures

[PureWeen]

🔍 Multi-Model Code Review — PR #607

Reviewed by: 3 independent reviewers with adversarial consensus
Diff reviewed: +117/−4, 2 files (2 commits)
CI Status: ⚠️ No checks configured for this branch

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v1 Findings — Status After Commit c9bcc64e

#	Finding	v1	v2
1	Empty-name fallback removed during promotion	🟢	✅ FIXED — if (string.IsNullOrWhiteSpace(name)) guard added in both paths + test
2	Undocumented promotion vs creation naming asymmetry	🟢	✅ FIXED — inline comment added at creation fallthrough + CreationPath_UsesFolderBasename test
3	Test paths don't exist on disk (implicit contract)	🟢	✅ FIXED — "path need not exist on disk" comments added to both test methods

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v2 Re-Review (commit c9bcc64e)

All 3 findings from v1 are resolved. The fix commit adds:

Production code (CopilotService.Organization.cs):

• Defensive IsNullOrWhiteSpace fallback in both ReconcileOrganization and PromoteOrCreateLocalFolderGroup
• 4-line comment documenting the intentional asymmetry at the creation-path fallthrough
• Debug messages now include group name for better diagnostics

Tests (SessionOrganizationTests.cs):

• CreationPath_UsesFolderBasename — pins the creation path behavior (folder basename used for new groups)
• FallsBackToFolderName_WhenGroupNameIsEmpty — verifies corrupt empty-name groups are repaired
• "Path need not exist on disk" notes on both existing regression tests

No new issues found in the fix commit. The changes are minimal, correct, and well-tested.

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Recommended Action

✅ Approve

All findings addressed. 3479 tests pass, 0 failures. The fix correctly prevents group name destruction during promotion while maintaining defensive fallbacks for corrupt state.