refactor: decompose fetch_latest_tag into focused helpers with module-level constant

[Copilot]

fetch_latest_tag was a ~40-line function mixing repo creation, network I/O, output parsing, and tag selection — violating the project's small-functions / no-magic-strings guidelines.

Changes

• TEMP_DIR_PREFIX constant — lifts "foc-devnet-tag-probe-" out of TempBareRepo::create into a module-level constant
• fetch_refs(repo, url, branch) — isolates the git fetch --tags --filter=blob:none invocation
• list_merged_tags(repo, branch) — isolates git tag --merged … --sort=-creatordate, returns raw stdout
• pick_first_tag(stdout, branch, url) — isolates first-non-empty-line selection

fetch_latest_tag becomes a 5-line orchestrator:

fn fetch_latest_tag(url: &str, branch: &str) -> Result<String, Box<dyn std::error::Error>> {
    info!("Fetching newest tag on branch '{}' from {}", branch, url);
    let repo = TempBareRepo::create()?;
    fetch_refs(&repo, url, branch)?;
    let stdout = list_merged_tags(&repo, branch)?;
    pick_first_tag(&stdout, branch, url)
}

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