docs: simplify and update README for v0.3.0

[justin-carper]

• Fix stale opencode version requirement: 1.16+ → 1.17+ (matches actual @opencode-ai/plugin: ^1.17.1 dep)
• Remove implementation-detail sections (How it works provider/plugin table)
• Trim session reuse section: keep scenario table, cut cache-implications wall
• Trim MCP section: keep mapping table, cut OAuth deep-dive
• Remove redundant Provider vs. delegation tools and Behavior & limitations sections
• Add OPENCODE_CURSOR_DEBUG and OPENCODE_CURSOR_SIDECAR to the env var table
• Simplify opening pitch and security callout