fix(firehose): update existing publishing PR comments when no packages are ready to publish (#449)

## Rationale
When a PR initially has a publishable version (e.g. `4.0.1-dev`) and firehose posts the `Package publishing` comment, but subsequently updates the package to a `-wip` version (e.g. `4.0.1-wip`), `VerificationResults.hasSuccess` becomes `false`.

Previously:
- In `lib/firehose.dart`, saving `existingCommentId` to `./output/commentId` and generating `./output/comment.md` was guarded strictly by `if (results.hasSuccess)`. When `hasSuccess` was `false`, neither file was written.
- As a result, the existing `Package publishing` comment with outdated `ready to publish` status and release links was never updated and remained on the PR indefinitely.

## Changes
- **`lib/firehose.dart`**: Save `existingCommentId` to `./output/commentId` unconditionally whenever an existing publishing comment is found. When `results.hasSuccess || existingCommentId != null`, write `./output/comment.md` with the updated `$markdownTable` (omitting the `"If you have publishing permissions..."` description when no packages are ready to publish).
- **`lib/src/github.dart`** & **`lib/src/local_github_api.dart`**: Add `deleteComment` helper to `GithubApi`.
- **`bin/comment.dart`**: Add `delete` subcommand (`firehose:comment delete --comment-id <id>`).
- **`test/firehose_test.dart`**: Add CLI argument validation and exit code tests for `comment delete`.
- **`CHANGELOG.md`**: Note that existing publishing PR comments are updated with accurate status when packages are no longer ready to publish.