docs: update documentation for subAgent tool and compaction

[avoidwork]

Description

Updates documentation across README.md and docs/ to cover the new subAgent tool and compaction tool features.

Changes

• README.md — Added subAgent to Built-in Tools table, process.subAgent to Config Reference, and new environment variables section
• docs/OVERVIEW.md — Added Sub-Agent section with features, configuration, and file descriptions
• docs/FLOWS.md — Added Sub-Agent Tool Execution Flow with call chains and helper functions
• docs/TUTORIAL.md — Added context compaction and built-in tools mentions

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

N/A — documentation-only change.

Coverage

• 100% line coverage maintained (no code changes)

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied