Add OpenCode terminal launch support

[calvintvu]

Context

OpenCode sessions appeared in coSlash but could not use the existing terminal resume and handoff actions. This adds launch parity with Claude Code and Codex while keeping handoff context invocation-scoped and opening new sessions without an automatic prompt.

Changes

• Resume an exact OpenCode session in its recorded working directory with vendor-specific session ID validation.
• Open blank OpenCode sessions with handoff context loaded from a secured temporary instruction file that is cleaned up after the TUI exits.

Test

• cd collector && make test - passed
• cd collector && make check - passed
• cd frontend && npm test - passed
• cd frontend && npm run lint - passed with two pre-existing warnings
• cd frontend && npm run format:check - passed
• Manual: Resumed an existing OpenCode 1.18.16 session and opened a fresh handoff session through the launch API; both returned HTTP 204.