Document Base agent workflow guardrails

[codeforester]

Summary

• Add Base-native guidance for adopting external agent workflow ideas without importing a whole third-party methodology.
• Document root-cause debugging, fresh completion evidence, worktree lifecycle guardrails, review-feedback handling, and skill-authoring rules.
• Tighten the PR checklist so validation evidence and regression/reproduction proof are called out explicitly.

Issue

Closes #500
Closes #501
Closes #502
Closes #503
Closes #504

Validation

• git diff --check
• test -f skills.md
• grep -q '^# Skills$' skills.md
• grep -q '^## Add a basectl subcommand$' skills.md
• grep -q '^## Add a Bash command$' skills.md
• grep -q '^## Add a Bash library$' skills.md
• grep -q '^## Add a Python CLI feature$' skills.md
• grep -q '^## Add or change artifact setup$' skills.md
• grep -q '^## Change shell startup behavior$' skills.md
• grep -q '^## Release or Homebrew-facing changes$' skills.md
• No BATS or Python tests were run; this is a documentation-only change.

Demo Impact

None. Documentation/process adoption only.

Notes

Adopted: Base-specific debugging, verification, worktree lifecycle, skill-authoring, test-first, and review-feedback guardrails.

Adapted: Superpowers concepts were translated into Base's existing AGENTS.md, skills.md, docs/github-workflow.md, docs/testing.md, and PR template structure.

Rejected: No plugin vendoring, no .worktrees/ convention, no docs/superpowers/* layout requirement, and no strict TDD rule for every docs-only or configuration-only change.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>.
• PR is scoped to one issue, unless a documented multi-issue exception applies.
• PR body explains what changed and how it was validated.
• Validation commands were run from the current checkout or worktree, or unavailable checks are explained.
• Relevant BATS and Python tests pass, or none are relevant for this docs-only change.
• Bug fixes include regression proof or a documented reproduction when practical, or the PR is not a bug fix.
• Documentation is updated when behavior or user-facing commands change.
• PR includes Fixes #<issue> or Closes #<issue> when it should close the issue.
• Demo Impact is meaningful for needs-demo work, or explicitly says None.