BACK-695 - Skip fetch when the requested Git remote is absent

[Lingikaushikreddy]

When a repository has an upstream remote but no origin, task reads try to fetch the missing origin and print a Git error. This change checks that the requested remote exists before fetching it, using the existing remote lookup. Repositories with that remote and offline mode keep their current behavior.

Fixes #1020.

Adds a regression for an upstream-only repository and updates existing fetch tests. The contributor reproduced the plain search error; the reported TUI display problem was not reproduced directly. CI passed on this head: Linux, macOS, and Windows tests and compiled smoke checks, plus the Nix package build. Type checks and formatting/lint checks passed in CI.

[Lingikaushikreddy]

Follow-up on the one test I flagged. I had only run board-tui-move.test.ts › "freezes the move set and ignores Escape…" in isolation after my change, which was weak evidence for calling it unrelated.

I've now run the full suite on unmodified main (c0ec546, without this branch): 2871 pass, 1 fail, and the failure is that same test. So it fails under full-suite load independently of this change.

Also, to be exact about the repro in the description: I reproduced the error text in backlog search --plain output, which is the same refreshRemoteRefsForTaskRead() path the reporter saw corrupting the board TUI. I did not reproduce the corrupted board itself, so treat that part as inference rather than something I measured.

[MrLesk]

@codex review

Alex's agent

[radupotop]

Hi Alex, is the codex review stuck? Would be great to have this reviewed and merged in. Thanks!