MAINT: Delete merged and abandoned remote branches

Problem

The repository has 41 remote branches, the oldest dating from 2014; roughly 28 predate 2026 and appear to be either merged release branches (rel-0.8.0, rel-v0.8.1, prep-v0.9.0, prep-v0.10.0, rls/v0.11.1, release-v0.11.3, rls-v0.11.4, v0.7.2, …) or long-abandoned feature work (alternate_optimization_growth 2014, lqcontrol-docstring 2015, tauchen_returns_mc 2016, sl/rouwenhorst 2017, update-fetch-utility 2017, update-numba 2020, …).

Note: update-fetch-utility (2017) may contain prior work relevant to the fetch_nb_dependencies fix in this milestone — review it before deleting.

Deleting a branch does not delete merged commits, and unmerged work remains recoverable from the reflog and any associated PR — but check for unmerged commits before each deletion rather than after. Enabling "Automatically delete head branches" in repository settings prevents the merged-branch half from recurring.

Acceptance criteria

• Every branch confirmed merged, or its unmerged commits captured in an issue, before deletion
• update-fetch-utility reviewed against the fetch fix first
• Merged release and feature branches deleted; "Automatically delete head branches" enabled
• Remaining branch count and the reason each survivor is kept recorded in this thread

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Start with the remote branch list and the audit reference commit 28d4b3b. Review update-fetch-utility against the fetch_nb_dependencies fix, then verify whether each candidate branch is merged and capture any unmerged commits before deletion. Done means the eligible branches are removed, automatic head-branch deletion is enabled, and the remaining branches and retention reasons are recorded in the issue.