Extend repo checks with GitHub agent-readiness validation

[codeforester]

Goal

Add a Base-owned check path that reports whether a repository is ready for GitHub-centered agentic implementation work.

Background

basectl repo check --agent-guidance currently checks the optional guidance files that repo agent-guidance creates. Base now also has Copilot instructions, an implementation issue template, and a Copilot cloud-agent setup workflow in the Base repository itself. A reusable agent-readiness check should make those expectations explicit for Base-managed repositories without forcing every repository to adopt every provider-specific file.

Scope

• Extend basectl repo check with a focused option such as --agent-ready, or extend --agent-guidance if that remains the clearer public contract.
• Check for the maintained guidance files required by the chosen agent-ready contract.
• Include GitHub-specific files only when the contract has opted into GitHub-hosted agent readiness.
• Print clear missing-file summaries and fix commands, for example basectl repo agent-guidance ... or basectl repo init ... --agent-ready.
• Update docs, command reference, help, completions, and AI context for any public option or file-set change.

Acceptance Criteria

• The new check succeeds for a repository containing the approved agent-ready file set.
• The check fails with actionable output when required files are missing.
• Existing repo check behavior remains unchanged when the new option is not passed.
• Existing repo check --agent-guidance behavior remains backward compatible or is intentionally documented as expanded.
• Tests cover success, missing files, no-overwrite fix hints, help, and completions.

Validation

• Relevant BATS coverage in cli/bash/commands/basectl/tests/repo.bats or a command-specific repo-agent test.
• Help/completion tests if the option surface changes.
• git diff --check.

Non-Goals

• Do not verify live Copilot subscription, billing, model access, or organization policy.
• Do not run hosted agents from this command.
• Do not require provider-specific files for repositories that only want provider-neutral guidance.
• Do not overwrite existing repo-local guidance.

Project Fields

• Priority: P2
• Status: Backlog
• Size: M
• Area: CLI
• Initiative: Contract Hardening

Agent Assignment

Copilot candidate after triage. This issue is suitable once the agent-ready file contract is settled.