Migrate backlog managers to a shared backlog-management skill

## Summary

Migrate the platform-specific backlog-manager instruction files into a single shared, platform-agnostic `backlog-management` skill with compact per-platform reference files, so that backlog planning conventions live in one authoritative place and are consumed by the ADO, GitHub, and Jira backlog prompts and agents.

Analysis showed the three platform planning specs share a large structural core (planning-file lifecycle, directory conventions, requirements, template skeletons, planning-type enum, scope normalization, reference-ID scheme, similarity assessment, autonomy tiers, sanitization). Only the tool/command catalog, field vocabulary, ID prefix, action verbs, and GitHub community communications differ per platform. A single shared skill can span the per-platform collections, so the target is one shared skill with per-platform reference files rather than three separate skills.

## Approach

Delivered incrementally, proving the pattern on Jira first:

- **Jira increment (first):** Build the shared `backlog-management` skill core plus a Jira platform reference; reduce the Jira planning instruction to a thin pointer that retains its activation glob; rewire the Jira prompts, agents, and collection membership to the skill; regenerate plugin, extension, and docs artifacts.
- **GitHub increment (deferred):** Add a GitHub reference file, trim/rewire GitHub consumers, register the skill in the GitHub collection.
- **ADO increment (deferred):** Add an ADO reference file, trim/rewire ADO consumers, register the skill in the ADO collection.

Authoring runs under the HVE Builder authoring, review, and validation standards.

## Out of scope

- The planners' backlog-templates skill (serves the planning agents, not backlog managers).
- Shared story-quality and community-interaction guidance (broader constituencies).
- ADO pull-request and build-info instructions.
- Consolidating backlog orchestration into the focused Jira CLI-tool skill.

## Acceptance

- Shared `backlog-management` skill with a Jira reference exists.
- Jira planning instruction reduced to a thin pointer retaining its activation glob.
- Jira prompts, agents, and collection YAML reference the skill.
- Generators and validation pass; no dangling references remain.