feat(opencode): add system prompt debug command

[Rexarrior]

Issue for this PR

Related to #24990, #39033, and #33333.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a local CLI debug command:

opencode debug prompt

The command prints the resolved effective system prompt for the current project, agent, and model. It supports optional --agent and --model flags.

This is useful when debugging prompt-affecting customization layers such as AGENTS.md, opencode.jsoninstructions, skills, MCP instructions, custom agents, and experimental.chat.system.transform plugins.

The implementation uses the same existing system prompt services used by normal request assembly:

• SystemPrompt.provider(model) for the model-family base prompt
• SystemPrompt.environment(model) for environment context
• Instruction.system() for AGENTS.md and configured instruction files
• SystemPrompt.mcp(agent) for MCP instructions
• SystemPrompt.skills(agent) for the available skills block
• experimental.chat.system.transform for plugin transformations

This is intentionally narrower than a full context dump or export feature. It does not persist anything to opencode.db, does not change transcript export behavior, does not add a TUI surface, and does not dump the full request/message array.

How did you verify your code works?

• Ran bun typecheck in packages/opencode
• Ran bun --conditions=browser ./src/index.ts debug prompt --help
• Ran bun --conditions=browser ./src/index.ts --pure debug prompt --model opencode/big-pickle
• Verified the real command output included the environment block, instruction files, and available skills block
• Ran bun run build --single --skip-install --skip-embed-web-ui
• Ran built binary smoke test: dist/opencode-darwin-arm64/bin/opencode --version
• Ran built binary command check: dist/opencode-darwin-arm64/bin/opencode debug prompt --help
• Pre-push hook ran bun turbo typecheck successfully

Screenshots / recordings

N/A. CLI-only change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[Rexarrior]

Hi! Friendly bump on this PR. The implementation is locally verified, but the required CI workflows are still waiting for maintainer approval. Could someone approve them and take a look when convenient?