Restore compatibility with OpenCode 2.0.6

[d3cker]

The previous beta SDK probes /api/health, which OpenCode 2.0.6 no longer exposes. Management commands and owner keepalive therefore fail even when an existing dispatcher still appears to be polling. This change updates the plugin to the 2.0.6 service and session APIs.

• Pin the client, plugin, and theme packages to 2.0.6. Use server.info for owner identity, session.update for keepalive, and resume: false when interrupting sessions.
• Replace obsolete headless activation instructions with plugin.list scoped to the project location. Update the README requirement, installation guidance, architecture, and affected workflow diagrams.
• Add integration coverage using the real SDK against a local HTTP server, including service discovery, RPC routing, owner keepalive, PID mismatch handling, and interruption request serialization.

Validation:

• npm run check: lint, type checking, 215 tests, native TUI rendering, and build passed.
• Isolated package installation passed; all eight Mermaid diagrams and 92 local documentation links validated.
• Deployed candidate commit e28cb69 to the test server running OpenCode 2.0.6. Verified active plugins, fresh discovery, successful management commands, and zero scheduler failures in megachlast_pvp, yoomp-, and opencode-automation-test.
• Verified maintenance-session updates, the live /botstatus TUI, and a successful isolated session using the configured model. Removed that test session afterward.
• Preserved all 17 tasks and 20 worktrees; configuration and worktree contents are unchanged. A deployment backup was retained on the server.

This PR targets devel. It does not publish a release or change the plugin version. The documented compatibility baseline is OpenCode 2.0.6; compatibility with the earlier beta SDK is not claimed.