test(switch_picker): fix Windows prompt-echo-lag flake; forbid retries

[max-sixty]

Removes a Windows-only false-positive on the advisory affected tests (windows) leg so it can eventually be promoted to a required gate.

test_switch_picker_alt_x_keeps_unmerged_branch_row flaked on the advisory Windows leg while the required test (windows) leg passed on the same SHA. From the captured CI frame, the panic was the content assertion, not the exit code: the old assertion counted the branch name across skim's query-echo prompt line plus the data row (>= 2), and under Windows CI load the prompt echo trails its keystrokes — the final character can still be unrendered once the row is already on screen — collapsing the count to 1.

The fix gates on the gutter-prefixed cursor row. A local branch with no worktree renders the / gutter (BranchScope::Local), so / unmerged-orphan names the data row and excludes skim's gutterless prompt, and it renders from collect data so it's immune to echo lag. This is the same robust idiom the sibling alt-x tests already use.

Also adds a "No Retries" section to tests/CLAUDE.md: worktrunk sets no nextest retries and writes no retry loops, with the two root-fix paths (racy assertion → make it deterministic; resource pressure → bound concurrency).

This does not touch the separate, lower-rate STATUS_DLL_INIT_FAILED (-1073741502) flake, which is Windows process-spawn resource exhaustion under concurrent load rather than a test-logic bug. The "No Retries" section documents the deterministic lever (bounding Windows test concurrency) for whenever the advisory leg is promoted.

The picker fix targets Windows-specific timing and passes on macOS, but the flake itself can only be confirmed gone on the Windows CI legs.

This was written by Claude Code on behalf of max