feat(scripts): collect PR URLs and prompt once at end of run

[sksizer]

Summary

Stops the update scripts from opening browser tabs the moment each PR is created. Instead, URLs are collected and the user is asked once at the end of the run whether to open them.

• New `scripts/lib/pr_prompt.sh` helper:
  • `pr_prompt_extract_url ` — pulls the first `https://github.com/.../pull/N\` URL out of a blob
  • `pr_prompt_finalize <urls...>` — lists URLs and, if stdin/stdout are a TTY and `PR_PROMPT_SUPPRESS` is unset, prompts `Open PR(s) in browser now? [y/N]` and opens via `open` / `xdg-open` / `cmd.exe start`
  • Honors `PR_PROMPT_SUPPRESS=1` to disable the prompt entirely (used by parent drivers and CI)
• Single-repo scripts (`bring_up_to_date.sh`, `cargo_update.sh`, `template_backport.sh`): drop their own `open_url` and call the helper instead. Run standalone, you get one prompt at the end.
• `*_all` drivers (`bring_up_to_date_all.sh`, `cargo_update_all.sh`, `template_backport_all.sh`): `export PR_PROMPT_SUPPRESS=1` so children stay quiet, then harvest URLs from per-repo logs and call `pr_prompt_finalize` once at the very end. `bring_up_to_date_all` now `tee`s child output to per-repo log files (it previously dropped output straight to terminal) so URLs can be collected.
• Non-interactive runs (CI, piped output) skip the prompt automatically and just print the URLs.

Test plan

• `just cu --execute path/to/repo` prints PR URL and prompts once at end
• `just cua --execute` runs across all downstream repos, lists every PR URL, prompts once
• Replying `y` opens all collected URLs; replying anything else exits cleanly
• `PR_PROMPT_SUPPRESS=1 just cua --execute` produces no prompt
• Piping the script through `tee` does not hang waiting for input