fix(plugin): restore automatic feedback monitor

[benvinegar]

Summary

• restore the Claude Code feedback monitor on versions that reject shell-form ${user_config.*} interpolation
• hand URL/token configuration to sideshow watch through private, per-session files and child environment variables instead of shell text or argv
• isolate concurrent sessions, consume successful handoffs immediately, and clean unused handoffs at session end
• bump the plugin to 0.1.1 so installed copies receive the fix
• clarify automatic Monitor delivery and MCP/CLI fallback guidance

Closes #242.

Validation

• claude plugin validate --strict ./plugin
• focused plugin tests, including concurrent-session isolation, cleanup, fallback, output forwarding, and spawn failure
• real interactive Claude Code 2.1.233 smoke test: browser comment → monitor event → Claude notification, with MCP not required for delivery
• Monitor-unavailable lifecycle smoke test: SessionEnd removed the unconsumed handoff
• npm test
• npm run coverage
• npm run typecheck
• npm run lint
• npm run format:check
• npm run security:audit
• npm run test:worker
• Node 22 focused plugin tests

This PR description was generated by Pi using gpt-5.6-sol