ao spawn --branch <invalid format> returns opaque 500 instead of 400 (residual of #152 Bug 3)

[codebanditssss]

Bug

ao spawn --branch "<invalid name>" (a branch name rejected by git check-ref-format) returns an opaque Internal server error (INTERNAL_ERROR) 500 instead of a 400 validation error.

Analyzed against: 3e64c15 | Confidence: High

Reproduction

ao spawn --project <p> --branch "bad branch name!!" --prompt x
# → Internal server error (INTERNAL_ERROR)

Root Cause

backend/internal/adapters/workspace/gitworktree/workspace.go validateBranch returns fmt.Errorf("gitworktree: invalid branch %q: %w", branch, err) — an untyped error. toAPIError (backend/internal/service/session/service.go) has no matching sentinel, so it hits default and the envelope collapses it to INTERNAL_ERROR 500.

This is the residual of #152 Bug 3: that RCA added ErrWorkspaceBranchNotFetched and ErrWorkspaceBranchCheckedOutElsewhere (now mapped to 400/409), but the invalid-format case — the very first check in validateBranch — was never given a sentinel, so it still 500s.

Fix

Add a ports.ErrWorkspaceBranchInvalid sentinel (mirroring the other two), wrap it in validateBranch, and map it in toAPIError → apierr.Invalid("INVALID_BRANCH", ...). The 400 carries git's own reason for the user.

Impact

User-controllable input (--branch) produces a confusing 500. Completes the "no opaque 500s on workspace branch errors" set from #152. Labels: bug, priority: medium.