enhancement: migrate nested basectl command parsing leaf by leaf

[codeforester]

Context

Follow-up implementation track from #2127 and #2122. The migration matrix groups gh*.sh, repo*.sh, and setup_diagnostics_fallback.sh because their nested subcommands own different grammars and may forward GitHub or tool options.

Scope

Create and execute a leaf-by-leaf parser migration plan that:

• records runtime-help and argv baselines for each selected leaf
• identifies Bash-owned options, nested subcommands, forwarded options, positional rules, and shell-stateful behavior
• migrates only the selected leaf grammar at a time, using base_arg_parse where its contract fits
• preserves help precedence, forwarded GitHub/tool arguments, duplicate and equals-form behavior, status codes, stdout/stderr ownership, and side-effect boundaries
• adds focused regression coverage before each leaf changes
• updates the migration matrix as leaves are completed

Start with the smallest self-contained leaf and keep each implementation PR reviewable. The tracking issue may remain open while successive leaf PRs land.

Coordination

Coordinate with #2117 for repo.sh ownership splitting; this issue does not authorize or require that structural refactor. Keep command metadata centralization in #1606 out of scope.

Out of scope

Do not rewrite gh.sh or repo.sh wholesale, change GitHub workflows or APIs, or alter shell-stateful behavior without a separate contract and tests.

Related: #2127

[codeforester]

The first leaf implementation is published as PR #2149, migrating through with runtime-help and option-looking-value coverage. This PR intentionally does not close #2144; the remaining nested leaves stay tracked here for successive PRs. Local validation: 62/62. No merge performed.

[codeforester]

Correction: the first leaf implementation is published as PR #2149, migrating gh issue readiness through base_arg_parse with runtime-help and option-looking-value coverage. This PR intentionally does not close #2144; the remaining nested leaves stay tracked here for successive PRs. Local validation: gh.bats 62/62. No merge performed.

[codeforester]

Completed the next self-contained #2144 leaf in PR #2184, merged as 734a458.\n\nThis migrates the basectl repo clone command to base_arg_parse while preserving its one-repository grammar, help precedence, duplicate options, dry-run output, owner inference, destination safety, and GitHub delegation. The migration matrix and focused regressions were updated.\n\nValidation: repo BATS 136/136; full BATS 778/778; contract suite passed; ShellCheck and syntax passed; basectl repo check passed; all 20 hosted checks passed. #2144 remains open for the remaining nested parser leaves.