fix: repo directory name collisions and custom directory support

[chriswritescode-dev]

Summary

• Allow users to specify a custom directory name when adding a remote repo, preventing collisions when cloning a fork of an already-cloned repo (e.g., forker/repo when owner/repo exists)
• Add shared validation and normalization utilities (shared/src/utils/repo.ts) for directory name sanitization, URL normalization, and path safety, used by both frontend and backend — replaces duplicated sanitizeWorkspaceAliasSegment in backend
• Backend validates directoryName before using it as a filesystem path (rejects path separators, dot-dot segments, absolute paths, invalid characters)
• Frontend detects directory collisions by normalizing both existing and new repo URLs (SSH/HTTPS/shorthand), shows inline warning, and disables submit until resolved
• Move SSH credential setup after DB record creation to avoid premature setup when repo already exists

Validation

• pnpm lint:frontend
• pnpm lint:backend
• pnpm test

Files

• backend/src/routes/repos.ts | 4 +-
• backend/src/services/repo.ts | 32 ++++-----
• frontend/src/api/repos.ts | 1 +
• frontend/src/components/repo/AddRepoDialog.tsx | 88 +++++++++++++++-
• shared/src/schemas/repo.ts | 1 +
• shared/src/utils/index.ts | 1 +
• shared/src/utils/repo.ts | 82 +++++++++++++++ (new)
  7 files changed, 187 insertions(+), 29 deletions(-)