docs: track dependency upgrade progress and internal link guidance

[simonrelet]

Summary

• Added a progress tracker to the dependency upgrade plan so upgrade execution status is visible at a glance.
• Added internal links from each progress row to the corresponding phase section for quicker navigation.
• Updated documentation-writing guidance to require co-located reference definitions for internal section links in long planning documents.

Context

• This is documentation-only work for planning and process consistency; no runtime code paths were changed.
• The tracking table enables incremental updates as each upgrade phase progresses.
• Manual verification focused on link integrity and markdown readability in raw format.

Changes

• Updated .github/instructions/docs-writing.instructions.md to require internal section link references directly below target headings in long planning documents.
• Added a Progress table in docs/plans/dependencies-upgrade.md with phase-level statuses and reference-style internal links.
• Moved per-phase reference definitions in docs/plans/dependencies-upgrade.md to sit directly below each corresponding section heading.
• Marked Standalone upgrades as Complete in docs/plans/dependencies-upgrade.md.

Risks and mitigations

• Risk: Internal heading anchor slugs can drift if headings are renamed.
  Mitigation: Reference definitions are now co-located with each target heading, so updates are localised and easier to maintain.
• Risk: The progress tracker can become stale if not updated during upgrades.
  Mitigation: The plan includes an explicit instruction to update status as work starts and completes.

Checklist

• Added in-code documentation (wherever needed). (N/A: docs-only changes)
• Ran the linter to ensure style guidelines were followed. (Not run for this docs-only change)
• Reviewed the changes for regressions.
• Manually tested the changed behaviour.