fix(cache): report prune removal failures accurately

[cedric013]

Summary

• Track failed BM25/graph cache prune removals explicitly.
• Count removed entries and freed bytes only after filesystem deletion succeeds.
• Surface failed removals in cache prune and doctor output.
• Add regression coverage for failed BM25 directory removal.

Closes #1281

Tests

• rustfmt --edition 2024 --check src/cli/config_cmd.rs src/doctor/fix.rs tests/index_scoping_scenarios.rs
• cargo test -q --test index_scoping_scenarios prune_bm25_counts_failed_remove_without_freed_bytes (cancelled after hanging in this workspace; CI will run the full suite)