feat(github): support @bgagent comment-triggered PR iteration

## Component

CDK GitHub adapter, orchestration, agent workflows

## Describe the feature

GitHub Issues should support reviewer follow-up comments that trigger ABCA to iterate on an existing PR, matching the Linear and Jira parity `@bgagent ` flow.

When a human comments on a GitHub issue with `@bgagent `, ABCA should find the active or most recent PR-producing task for that issue and run `coding/pr-iteration-v1` against that PR.

## Use case

After ABCA opens a PR from a GitHub issue, reviewers may add follow-up instructions on the originating issue. They should not need to leave the issue and manually submit a new task or comment on the PR.

## Proposed solution

Add a GitHub issue-comment trigger path:

- Subscribe the GitHub webhook setup to `issue_comment` events in addition to `issues`.
- Parse comment bodies for an `@bgagent` mention and extract the remaining instruction.
- Ignore comments authored by ABCA/the app user to prevent self-trigger loops.
- Resolve the GitHub issue to an existing ABCA task/PR using `channel_source === "github"` and GitHub issue metadata.
- Create a new task with `workflow_ref: "coding/pr-iteration-v1"`, the resolved repo, PR number, and reviewer instruction.
- Attribute the iteration to the linked GitHub user when available; otherwise use the original task owner rather than dropping the request.
- Post an immediate GitHub acknowledgement comment, then rely on platform-side final-status fanout for the terminal cost/outcome comment.
- For orchestrated GitHub child issues, route the iteration to the child task/PR and let orchestration re-integration restack dependents when available.

## Acceptance criteria

- A GitHub issue comment `@bgagent update the README too` on an issue with an ABCA PR creates a `coding/pr-iteration-v1` task for that PR.
- The GitHub issue receives an acknowledgement comment quickly after the trigger is accepted.
- The terminal GitHub final-status comment includes the iteration outcome and cost once the run finishes.
- Comments without `@bgagent`, comments from the ABCA app/bot, and unsupported comment events are no-ops.
- If no PR-producing ABCA task can be found for the issue, ABCA posts a clear GitHub comment instead of creating a task.
- Webhook redelivery does not create duplicate iteration tasks.
- Existing label-trigger behavior remains unchanged.

## Tests

- GitHub webhook tests cover comment parsing, bot/self-comment skip, no-PR fallback, idempotent redelivery, and successful iteration task creation.
- Task lookup tests cover resolving a GitHub issue to the correct prior PR task.
- Orchestration tests cover GitHub child-issue iteration metadata where applicable.
- Docs update the GitHub Issues setup guide with the required `issue_comment` event and usage examples.

## Out of scope

- Arbitrary comments as task triggers without an `@bgagent` mention.
- PR review comments or PR-native triggers; tracked separately.
- Natural-language routing from a parent issue to one of several child issues.
- GitHub Projects automation.

Start with the GitHub webhook setup and issue-comment trigger path, then trace the existing task lookup and orchestration entry points. Run the GitHub webhook, task lookup, and orchestration tests described in the issue; done means accepted mentions create one PR-iteration task, acknowledgements and final status are posted, no-op cases remain safe, and the setup guide documents issue_comment.

Written by the indexing model from the issue text.