feat: add universal agentic runtime

[wxai-space]

Summary

• replace the coding-only fixed path with one unified dynamic Agentic Loop while retaining fixed LightFlow workflows as an explicit option
• add auditable built-in web, Playwright/optional DrissionPage browser, Docker-only shell, MCP, Markdown Skills, scoped memory, SQLite FTS5/optional embedding RAG, goals, context compression, and bounded parallel subagents
• redesign the local UI as a streaming chat with follow-ups, steering, approval dialogs, agent/goal/tool visibility, screenshots, management panels, and diff output only when files change
• add durable event/tool/approval artifacts, exact-argument approvals, resource budgets, safe read-only degradation when Docker is unavailable, and recovery from invalid LightAgent tool-loop output
• preserve per-subagent traces and recover exhausted specialist loops with a tool-free evidence synthesis pass
• run Playwright/Drission backends on a dedicated browser thread so LightAgent's async dispatcher cannot conflict with synchronous browser APIs
• keep LightAgent 0.9.7 as the runtime; Agentica is design reference only and is not added as a dependency

Safety decisions

• shell and stdio MCP execute only through the task Docker container; there is no host-shell fallback
• browser sessions use temporary profiles and block private-network targets/downloads by default
• Playwright is the default browser backend; DrissionPage remains an optional dependency
• embedding RAG is disabled by default and falls back to FTS5 on provider failure
• local model credentials remain in the ignored lightworker.local.yaml and are not included in this PR

Verification

• pytest -q: 84 passed, 2 skipped (Docker daemon/image unavailable)
• ruff check .
• python -m compileall -q src tests
• uv lock --check
• uv build --out-dir /tmp/lightworker-dist
• live OpenAI-compatible task plus follow-up completed successfully
• Playwright opened https://example.com/, extracted content, and captured a screenshot using an ephemeral profile
• local /api/health and /api/capabilities checks passed on LightWorker 0.2.0
• live A-share analysis recovered two exhausted Research subagents (recovered: true) and completed successfully
• async-dispatched browser_open + browser_extract regression completed successfully against https://example.com/