Add fractal to 智能体 Agents

[ryanpettry]

Adds fractal to the 智能体 Agents section as item 53.

fractal is an Apache-2.0 runtime for hierarchical coding-agent loops. It supports recursive delegation, per-node Git worktrees, configurable limits, persistent SQLite state, and live terminal monitoring and steering.

Why

• Adds a recursive hierarchy model to the section's multi-agent frameworks and platforms
• Keeps delegated changes in separate Git worktrees
• Makes long-running workflows inspectable through persistent state and live controls

Validation

• git diff --check
• Confirmed the new entry follows item 52 as item 53
• Confirmed the repository URL returns HTTP 200
• Duplicate README, history, issue, and pull-request searches found no existing fractal submission

Disclosure: I am affiliated with Plasma AI, which maintains fractal.