fix(specgit): target delivery pull requests at dev #528

This was generated by AI during triage.

Why

specgit issue derives a draft pull request against the repository default branch (main). This repository requires feature/fix/chore deliveries to enter through dev; during #519 the generated PR #520 therefore targeted the wrong integration tier until manually retargeted.

The wrong base starts main-only acceptance work, bypasses the intended fast integration layer, and creates a merge-risk if it is not noticed immediately.

Scope

• Make the repository's SpecGit bootstrap path select dev as the delivery PR base.
• Prefer an upstream/configurable target-branch seam when available; otherwise extend the repository-local bootstrap wrapper introduced by chore(specgit): automate bootstrap around required harness specializations #521.
• Verify the base before the command reports success.

Out of scope: changing GitHub's default branch or weakening either branch ruleset.

Acceptance

• A new SpecGit delivery opened from an allowed delivery branch has baseRefName == "dev" without a manual gh pr edit step.
• The bootstrap fails clearly if it cannot verify or correct the PR base.
• Tests cover the configured dev target and preserve SpecGit's default behavior outside this repository-specific path.

Dependency

• Blocked by chore(specgit): automate bootstrap around required harness specializations #521's safe repository-local bootstrap seam.