Add JSON output and DEBUG logging to OpenCode engine

[Copilot]

OpenCode workflows lacked structured output and verbose logging for debugging workflow runs.

Changes

OpenCode Engine (pkg/workflow/opencode_engine.go)

• Added --format json for structured output parsing
• Added --log-level DEBUG for maximum verbosity
• Added --print-logs to stream logs to stderr in real-time

Shared Workflow (.github/workflows/shared/opencode.md)

• Updated Run OpenCode step with new CLI flags
• Updated documentation to reflect enhanced logging

Test Updates

• Fixed completions test for 5 engines (added opencode)
• Updated OpenCode version constant to 0.15.13

Result

OpenCode workflows now emit:

opencode run "$(cat /tmp/gh-aw/aw-prompts/prompt.txt)" \
  --print-logs \
  --log-level DEBUG \
  --format json

Enables programmatic log parsing, real-time debugging in Actions console, and better observability for workflow analysis.

Original prompt

Reference: https://github.com/githubnext/gh-aw/actions/runs/21047375867/job/60525148596#step:26:1

Review opencode and figure out how to get maximum logs, and structure JSON output

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