docs: add agent-oriented harness documentation

[07prajwal2000]

Summary

• Added concise agent-oriented docs for Fluxify, workflows, blocks, context, and the JavaScript API.
• Added detailed agent pages for Error Handler and Custom Block behavior.
• Added VitePress exclusion and synchronization for publishing .agent.md files under /agent-docs.
• Added an /agent-docs/index.json manifest for discovery.

Validation

• bun run docs:build
• git diff --check

Notes

• Human-facing documentation was not changed.
• Agent Markdown is excluded from VitePress page rendering and published as raw files.
• Pre-commit checks were skipped as requested.

Refs #271