feat: add setup-target-repo.sh label bootstrap

[yihanzhu]

Closes #5

What

Adds scripts/setup-target-repo.sh to bootstrap a target repo's loop labels
(ready, round-0..round-3, needs-human) with one command, and points the
docs at it.

Why

Onboarding a new target repo currently means hand-running a gh label create
loop from templates/repo-setup.md. Scripting the labels step makes adding a
repo one command, supporting the reusability goal in CLAUDE.md.

How

• scripts/setup-target-repo.sh
  • #!/usr/bin/env bash, set -euo pipefail, executable bit set.
  • Parameterized on <owner>/<repo> (no hardcoded repo); prints usage and
    exits non-zero if the arg is missing.
  • Idempotent: gh label create ... || gh label edit ... so re-runs on a
    repo that already has the labels update rather than fail.
  • After labels, prints the manual follow-ups it can't script (branch
    protection, CI workflow, Claude GitHub App, Codex reviewer) and points to
    templates/repo-setup.md.
• templates/repo-setup.md: labels step now references the script instead of
  the inline loop.
• README.md: scripts/ added to the Layout block.

How verified

• shellcheck scripts/setup-target-repo.sh → clean (exit 0), installed locally.
• bash -n parse check passes.
• Missing-arg path prints usage to stderr and exits 1.
• Structure check (all CI-required files present) still green.

Scope is label bootstrap only — no branch-protection / CI / GitHub-App / Codex
automation (out of scope per the issue).

[yihanzhu]

🤖 Codex reviewer (gpt-5.5) — automated cross-vendor review vs main (codex exec review):

Verdict: no actionable correctness issues. No blocking comments → ready for the human merge gate.

Verified during review:

• shellcheck clean + bash -n parse OK
• the gh label create … || gh label edit … idempotency approach is valid against the actual gh CLI
• doc updates (repo-setup.md, README) align with the script's usage
• consistent with CLAUDE.md conventions

Reviewer = Codex · coder = Claude — cross-vendor decorrelation. Converged at round-0 (no revisions needed).