Remove leftover branch git config when deleting local branches

[Copilot]

The "Delete local branches and remotes" command (pr.deleteLocalBranchesNRemotes) listed branches that no longer exist. getBranchDeletionItems builds its picklist from .git/config, and stale [branch "<name>"] sections (left behind when a branch is deleted out-of-band) keep those branches appearing.

Changes

• deleteBranches config cleanup — after unsetting the PR metadata keys, deleteConfig now also strips any remaining branch.<name>.* entries. This happens in the existing sequential config step (config ops can't be parallelized), scoped to the exact branch prefix so other branches/remotes are untouched, and logs rather than aborts on failure.
• Test mock — MockRepository gains unsetConfig, and deleteBranch now throws a realistic git "not found" error (with stderr) for missing branches.
• Tests — cover cleanup for both an existing branch and an already-deleted branch with stale config.

if (this.repository.unsetConfig) {
    const prefix = `branch.${branch}.`;
    const remaining = (await this.repository.getConfigs()).filter(config => config.key.startsWith(prefix));
    for (const config of remaining) {
        await this.repository.unsetConfig(config.key);
    }
}

Note: the electron test runner could not execute in the sandbox (VS Code download is network-blocked); tests type-check and the cleanup logic was verified in isolation.