docs: add DORA Metrics, Sprint, and Code Freeze terms to dictionary

[iamgerwin]

Summary

Add two new IT/software engineering terms to the dictionary that are relevant to modern development practices:

1. Sprint - Core Scrum term that is referenced in the dictionary but not defined independently
2. Code Freeze - Essential release management term used across development teams

Note: DORA Metrics was initially planned but already exists in the dictionary.

Rationale

• Sprint: While mentioned in the Scrum entry, Sprint deserves its own entry as it's one of the most commonly used terms in agile development.

• Code Freeze: A critical release management practice that developers encounter regularly, especially in enterprise environments.

Changes

• Append 2 new terms to docs/dictionary.md
• Maintain alphabetical ordering within sections
• Follow existing format (Category, Description, Related Terms)

Acceptance Criteria

• Sprint added to S section
• Code Freeze added to C section
• All entries follow existing dictionary format
• No existing terms modified or deleted