Add rake task to classify dead-shard pool repository members

What does this MR do and why?

Adds a read-only rake task (gitlab:pool_repositories:classify_dead_shard_members) that classifies member projects of sourceless pool repositories sitting on decommissioned Gitaly shards.

Per the Phase 2 remediation plan, 133 pool_repositories rows on dead shards still have ~172 member projects pointing at them. Before remediation, each member project must be classified by querying Gitaly to determine its on-disk state:

• Self-contained: repo is complete, no pool needed on disk
• Silently linked: repo is linked to an on-disk pool on its current shard (storage move replicated the pool but DB bookkeeping was incomplete)
• Broken: repo is missing or has missing objects

The rake task outputs a CSV with per-project classifications to guide the subsequent remediation steps.

Usage

# Omnibus (production)
sudo gitlab-rake gitlab:pool_repositories:classify_dead_shard_members \
  SHARD_NAMES=nfs-file01,...,nfs-file110,praefect-file01 \
  OUTPUT_FILE=/tmp/classification.csv

# Source / GDK (requires PostgreSQL to be running: `gdk start postgresql`)
bundle exec rake gitlab:pool_repositories:classify_dead_shard_members \
  SHARD_NAMES=nfs-file01,...,nfs-file110,praefect-file01 \
  OUTPUT_FILE=/tmp/classification.csv

Sample output (local GDK)

Local GDK has no dead-shard data, so the task completes with zero results — confirming it runs end-to-end without errors:

I, [2026-09-10T18:02:59.953167 #70808]  INFO -- : No matching shards found in database. Nothing to classify.
I, [2026-09-10T18:02:59.953502 #70808]  INFO -- :
I, [2026-09-10T18:02:59.953574 #70808]  INFO -- : === Classification Report ===
I, [2026-09-10T18:02:59.953592 #70808]  INFO -- : Self-contained: 0
I, [2026-09-10T18:02:59.953608 #70808]  INFO -- : Silently linked: 0
I, [2026-09-10T18:02:59.953623 #70808]  INFO -- : Broken: 0
I, [2026-09-10T18:02:59.953636 #70808]  INFO -- : Unknown/Error: 0
I, [2026-09-10T18:02:59.953657 #70808]  INFO -- : Total: 0
I, [2026-09-10T18:02:59.954613 #70808]  INFO -- : Classification complete. Results saved to /tmp/classification.csv

References

• Parent issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/616941
• Epic: https://gitlab.com/groups/gitlab-org/-/epics/19130
• Phase 1 cleanup rake task: !252707 (merged)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.