Prune stale log-retention index entries before enforcing max_log_files

[codeforester]

Problem

The .base-cli-log-index.json can contain paths for log files that no longer exist. prune_log_files() currently counts those stale entries when calculating excess_count, which can delete more real log files than max_log_files permits.

A concrete reproduction with three real logs, one phantom index entry, and max_log_files=3 deletes two real logs instead of one.

Scope

• Filter indexed paths to existing files before building pruning candidates and calculating excess_count.
• Keep the current log file eligible for retention even before it exists on disk.
• Add a regression test covering a stale/phantom index entry and asserting the configured real-file limit.

Acceptance criteria

• Stale index entries never consume a retention slot.
• Pruning leaves at most the requested number of real log files, including the current log.
• Existing index-based retention behavior remains covered.