Fix inconsistent repo picker: use dropdown in both new session and multi-agent flows

[StephaneDelcroix]

Bug

In 'New Session' the repository selector is a <select> dropdown, but in 'New Multi-Agent' it was a list of clickable buttons. This inconsistency is confusing.

Fix

Changed the multi-agent repo picker (Step 1 in SessionSidebar.razor) from a button list to a <select> dropdown matching CreateSessionForm:

• Multiple repos: <select class="ns-repo-select"> dropdown with placeholder option
• Single repo: <div class="ns-repo-label"> label (auto-advances to Step 2)
• No repos: Existing 'no repositories' message

Also added OnMultiAgentRepoChanged handler and auto-advance logic when there's exactly 1 repo.

Tests

Added RepoPickerConsistencyTests.cs (8 tests) verifying:

• Both forms use <select> with ns-repo-select class
• Multi-agent picker no longer uses button list
• Single-repo label behavior matches
• Auto-advance and placeholder option exist
• Handler exists for dropdown change event

All 2760 tests pass.

[PureWeen]

PR Review: Fix inconsistent repo picker (#404)

CI Status: ⚠️ No checks reported on this branch.

---

Round 1 Findings

#	Severity	Finding
F1	🟡 MODERATE	Vacuous test — SessionSidebar_MultiAgent_DoesNotUseButtonListForRepos used if (step1Match.Success) { ... } with no assertion when the regex didn't match, so the test passed silently on comment rename/restructure
F2	🟢 MINOR	Unreachable template branch — The else if (Count == 1) / ns-repo-label in SessionSidebar.razor is never rendered: StartAddMultiAgentGroup() auto-advances synchronously before Blazor renders

---

Fix Applied (commit 51ae507)

F1 fixed: replaced the if-guarded assertion with:

Assert.True(step1Match.Success, "Could not locate Step 1 section...");
Assert.DoesNotContain("worktree-item", step1Match.Value);

All 8 new tests pass after the fix.

---

Round 2 Findings (Post-Fix)

Finding	Status
F1 — Vacuous test	✅ FIXED
F2 — Unreachable ns-repo-label branch	🟢 STILL PRESENT (harmless — acceptable consistency tradeoff with CreateSessionForm; 5/5 models found no new issues)

---

Test Coverage

8 new snapshot-style tests cover: both forms use ns-repo-select, no button-list in step 1, single-repo label, auto-advance logic, placeholder option, and handler existence. Coverage is appropriate for a UI consistency fix.

---

Recommended Action: ✅ Approve

The core fix is correct and well-tested. The remaining minor item (dead ns-repo-label branch) is harmless dead code kept for template symmetry with CreateSessionForm.