Add small local-ergonomics recipes to the justfile

[travisennis]

Problem

Several recurring manual steps have no one-command wrapper:

• Targeted fast test runs compile and run every integration-test binary, each printing a "running 0 tests" block — want just test-one <filter> with --bin cake.
• Force-deleting unmerged scratch branches is blocked by the Bash command guard and there is no safe recipe — want just branch-rm <name> (safe delete with confirmation for unmerged branches).
• Dependabot PR triage (list with --json, local merge-conflict probe, gh run rerun trigger) is ad-hoc — want just dependabot-triage.
• Flaky failed CI jobs need repeated gh run rerun --failed round-trips interleaved with polling — want a rerun-and-watch helper.
• Removing a just recipe requires an ad-hoc rg sweep of docs, skills, and AGENTS.md — want just recipe-refs (lint backticked just <recipe> references against the justfile).

Proposed Solution

Add the recipes above. Each is independently shippable; split into separate issues if preferred.

Alternatives Considered

Leave the steps manual (status quo). The sandbox-compatible branch-delete workaround (git update-ref -d refs/heads/<branch>) is already in LEARNINGS.

Additional Context

Recorded in ~/.local/share/cake/DESIRES.md (2026-08-04: dependabot triage, flaky rerun; 2026-08-05: recipe-refs; 2026-08-10: test-one).