Add configurable log rotation naming via template

[pmonday]

Description

The current default log rotation naming uses a zero-padded sequence number pattern (e.g., audit-log.jsonl → audit-log.002.jsonl → audit-log.003.jsonl).

We need to allow users to configure the rotation naming convention via a template string passed to Rotate() or Log configuration.

Proposed Approach

Add a LogOption or configuration that accepts a template string with variables like:

• {{.Base}} - original filename without extension
• {{.Ext}} - file extension
• {{.Seq}} - zero-padded sequence number
• {{.Timestamp}} - rotation timestamp
• {{.PrevPath}} - previous log path

Example template: "{{.Base}}.{{.Seq}}{{.Ext}}"

Acceptance Criteria

• Add LogConfig struct or option function for NewLog
• Support template string in rotation naming
• Default template preserves current behavior ({{.Base}}.{{.Seq}}{{.Ext}})
• Document template variables in docs/rotation.md
• Add tests for custom naming patterns

Related

• Part of Issue Design audit log file rotation mechanism for regulatory compliance #2 (File rotation mechanism)