feat(aidd-vcs): feature-branch and optional worktree setup before implementation

Start with plugins/aidd-dev/skills/00-sdlc/SKILL.md and plugins/aidd-vcs/skills/00-repo-init/ to understand the existing branch and repository setup guidance. Review .aidd/config.yml and the issue's branch.base, branch.pattern, and mode requirements. Done means branch-only is the default, worktree mode reports real isolation or fails explicitly, existing work is not clobbered, and SDLC docs name a branch-setup capability.

Problem

SDLC requires a non-default branch before shipping, but nothing creates that branch or optional isolation. Implementation often lands on the default branch or a shared dirty tree and collides with parallel work.

Scope

• Skill for branch setup (semantic name e.g. worktree / branch-setup):
  • Modes from config/args: branch-only (default) | worktree | fail if requested mode unavailable.
  • Create feature branch from branch.base + branch.pattern (.aidd/config.yml) or args.
  • worktree mode only when git worktree is available; otherwise error or explicit fallback — never claim isolation that was not obtained.
  • Print path + branch for implement hand-off.
• v1 promise: implement on a non-default branch. Stronger “base tree unchanged” only when worktree mode succeeds.
• Optional pre-implement capability: SDLC docs use capability language only (no hardcoded plugin addresses — orthogonality).

Standalone value: Implement need not use the default branch if this skill runs first.

Acceptance criteria

• Default mode branch-only: non-default branch created/checked out for work; no worktree required.
• Mode worktree: when available, second worktree on feature branch; original tree can remain on base.
• Mode worktree when unsupported: non-zero or explicit fallback — never claims isolation it did not get.
• No clobber of existing branch/worktree without reuse flag.
• No commit/push unless opted in.
• SDLC docs reference a branch-setup capability, not a hardcoded plugin id.

Prior art in this repo

• plugins/aidd-dev/skills/00-sdlc/SKILL.md (L31) — “Never auto-branch; the caller sets a non-default branch before shipping”
• plugins/aidd-vcs/skills/00-repo-init/ — repo bootstrap, no feature worktree
• #413 — commit-on-branch guidance in project memory / VCS templates

Relations

Intended graph (same fields as GitHub parent / blocked-by / blocks). Formal GraphQL edges need maintainer write — see comment on #584.

Out of scope

• Remote concurrency scan (recheck child)
• Issue linking policy