Add workspace agent brief from workspace manifests

[codeforester]

Goal

Add a workspace-level agent brief that summarizes a Base workspace in a form suitable for issue-backed implementation work across multiple repositories.

Background

Base has workspace manifests and basectl workspace onboarding, which already report repository presence, optional repos, and onboarding state. Agentic coding raises a related but distinct need: before assigning a task, a human or hosted agent needs to know which repos are in scope, which repos have agent guidance/context, which commands validate the workspace, and which repositories are missing or local-only.

Scope

• Add a workspace agent brief surface, either as basectl workspace agent-brief or an agent-focused option on workspace onboarding.
• Read the existing workspace manifest and local repository state.
• Report per-repository agent-readiness signals such as baseline presence, agent guidance presence, .ai-context presence, and validation command availability where Base can infer them safely.
• Include text output for humans and JSON output for automation if consistent with existing workspace report patterns.
• Keep GitHub network calls optional or absent in the first slice unless the command explicitly asks for remote checks.
• Update README, workspace docs, command reference, and AI context if the command surface changes.

Acceptance Criteria

• The command reports a concise workspace brief for a manifest with complete, missing, optional, and local-only repositories.
• The output identifies which repos are ready for agent handoff and which need repo agent-guidance, repo check, or baseline work first.
• JSON output, if included, has stable keys and focused tests.
• Existing workspace onboarding behavior remains unchanged unless the new feature is an explicit mode.
• Tests cover complete, missing, optional, and partial/non-Base repository cases.

Validation

• Relevant Python tests under cli/python/base_projects/tests for workspace report behavior.
• Bash dispatch/help tests if a new subcommand or option is added.
• git diff --check.

Non-Goals

• Do not clone repositories automatically.
• Do not assign GitHub issues or start PR branches automatically.
• Do not require every workspace repository to use Base or GitHub.
• Do not add hosted telemetry or remote dashboard behavior.

Project Fields

• Priority: P2
• Status: Backlog
• Size: M
• Area: Workspace
• Initiative: Workspace Handling

Agent Assignment

Copilot candidate after triage. This issue is suitable if the first slice remains report-only.