feat(github): support bgagent:decompose and bgagent:auto planning labels

[ayushtr-aws]

Component

CDK GitHub adapter, orchestration, agent workflows, CLI

Describe the feature

GitHub Issues should support the same planning labels as Linear and the Jira parity work:

• bgagent:decompose: ABCA analyzes the GitHub issue, proposes a dependency-aware breakdown, posts the plan to GitHub, and waits for approval before creating/running child work.
• bgagent:auto: ABCA analyzes the issue, creates the breakdown, and starts the child work without an approval round.
• bgagent:help: ABCA posts a short comment explaining the supported GitHub trigger labels and creates no task.

Use case

A large GitHub issue may need to be split before execution. Users should be able to keep that planning workflow inside GitHub Issues with the same scope/cost control available in Linear/Jira.

Proposed solution

Extend the GitHub Issues label-trigger path to recognize label variants based on the configured base label:

• <label> keeps current single-task behavior unless the issue already has an orchestratable child graph.
• <label>:decompose creates a coding/decompose-v1 planning task, posts the resulting plan as a GitHub issue comment, and records a pending plan awaiting approval.
• <label>:auto creates the same plan and immediately seeds/runs the generated child graph.
• <label>:help posts a GitHub help comment and exits.
• Add per-repo GitHub mapping options for decomposition enablement and max sub-issues, mirroring Linear/Jira parity.
• Preserve GitHub channel metadata on generated child tasks so comments, final status fanout, and later graph extension work.

Acceptance criteria

• Adding bgagent:help to a GitHub issue posts a help comment and creates no task.
• Adding bgagent:decompose to an eligible GitHub issue creates a planning run and posts a proposed breakdown with cost/scope information and approval instructions.
• No implementation child tasks start until the plan is approved.
• Adding bgagent:auto creates a plan and starts the generated child graph without a human approval round.
• Per-repo caps reject over-large plans with a GitHub comment and no child tasks.
• Projects/repos without decomposition enabled reject :decompose / :auto with a clear GitHub comment.
• Existing plain-label GitHub behavior remains unchanged.

Tests

• GitHub processor tests cover label variant parsing, help no-op, decompose planning task creation, auto path, disabled-repo rejection, and cap rejection.
• Orchestration tests cover pending-plan persistence and generated GitHub child task metadata.
• CLI tests cover GitHub mapping flags for decomposition enablement and max sub-issues.
• Docs update the GitHub Issues setup guide with label variants and cap configuration.

Out of scope

• Running existing GitHub sub-issues/dependencies; tracked separately.
• Rich interactive plan revisions beyond the minimum approval path.
• Deterministic plan edit commands such as drop 3 or merge 1 2.
• GitHub Projects automation.
• Cross-repo decomposition.