The committed backlogs are migrated and the files retired

[sameera]

As a delivery lead, I want the existing backlog blocks moved onto issues and the files
deleted, so that no stub is recorded in two places.

Acceptance Criteria

• Given the 22 stub blocks marked proposed across the 7 feature backlogs, when
  migration completes, then each is an open backlog-labelled issue whose source line
  preserves the original date recorded in its block.
• Given the 12 promoted and 3 superseded blocks, when migration completes, then
  none of them has been refiled as an issue.
• Given a stub block whose original text carried multi-paragraph rationale (the 10 proposed
  entries under multi-repo-workspaces), when it is migrated, then that rationale is
  present in the issue body rather than truncated to the one-line goal.
• Given migration completes, when the docs tree is searched, then zero backlog.md
  files remain under the docs root's features/ tree.
• Given the files have been deleted, when the committed docs are searched for references
  to them, then no committed document links or points at a deleted backlog.md — the
  deletion never lands ahead of the references being repointed.

Notes

Sequencing: this story lands last. Deleting the files while either writer still appends to them
(#186, #187), or while promotion still globs for them (#188), recreates the
files on the next run; deleting them before the READMEs are repointed (#190) leaves
committed links dangling. That is why it is blocked by all four.

The multi-repo-workspaces backlog is the one that needs judgement rather than a mechanical
conversion: 10 of its proposed entries carry re-scoping rationale in prose that a slug-plus-goal
extraction would drop.

The already-promoted and superseded blocks are deliberately not refiled — git history is
sufficient provenance for them, and refiling them as pre-closed issues would add noise to exactly
the query this epic exists to make useful.