Add agent handoff report bundle for issue-backed work

[codeforester]

Goal

Add a local report bundle that packages the Base context an implementer or coding agent needs before starting issue-backed work.

Background

Base already has .ai-context, basectl export-context, basectl history --report, logs, check/doctor JSON, and GitHub issue/worktree helpers. These are useful individually, but agentic coding benefits from a single handoff artifact that says: what issue is being worked, what repo/workspace context applies, what validation commands are expected, what the recent local state says, and where the canonical instructions live.

Scope

• Add a command such as basectl report --agent-handoff or a narrower basectl gh issue handoff <number> if that better fits existing command ownership.
• Include links or embedded summaries for AGENTS.md, .ai-context/INDEX.md, exported context files, recent history report, check/doctor status, and issue metadata when available.
• Support a deterministic Markdown output path and an optional JSON descriptor if useful for automation.
• Redact or omit secrets, tokens, private env values, and excessive command output.
• Keep the artifact local by default; provider upload remains separate and covered by existing provider-adapter evaluation work.

Acceptance Criteria

• The command can produce a handoff Markdown report for the current Base repository and a specified issue number or local-only task label.
• The report lists canonical instructions, current branch/worktree state, issue metadata if supplied, validation guidance, and available context exports.
• The command fails closed or warns clearly when GitHub metadata cannot be fetched.
• Output is deterministic enough for tests after replacing timestamps or paths with fixtures.
• Documentation explains that the report is local and provider-neutral.

Validation

• Focused tests for report construction and redaction behavior.
• GitHub metadata failure fixture if the command reads issues.
• git diff --check.

Non-Goals

• Do not upload context to ChatGPT, Claude, Copilot, or another provider.
• Do not start an agent session.
• Do not include secrets, tokens, full logs by default, or private local notes.
• Do not replace basectl export-context or basectl history --report; compose them where appropriate.

Project Fields

• Priority: P2
• Status: Backlog
• Size: M
• Area: CLI
• Initiative: Adoption Polish

Agent Assignment

Human first. This issue crosses several existing context/report surfaces and should get a narrow design decision before implementation.