feat(cli): add --json flag for machine-readable output across all commands

[EndersonPro]

Summary

Add --json flag to all CLI commands to enable machine-readable JSON output for integration with scripts and other tools.

Changes

New Flag: --json

All commands now support --json to output results as formatted JSON:

Command	JSON Output
flutree version --json	{"version": "1.0.0"}
flutree list --json	Array of worktree objects
flutree config set/get --json	{"key": "...", "value": "..."}
flutree create --json	CreateResult object
flutree complete --json	CompleteResult object
flutree pubget --json	PubGetResult object
flutree clean --json	CleanResult object
flutree add-repo --json	AddRepoResult object
flutree update --json	UpdateResult object

Error Handling

When --json is passed, errors are also returned as JSON:

{
  "error": {
    "category": "input",
    "code": 2,
    "message": "Missing worktree name.",
    "hint": "Usage: flutree create <name> [options]"
  }
}

Files Modified

• cmd/flutree/main.go - Added --json flag to all commands + help documentation
• internal/domain/types.go - Added JSON struct tags to all result types
• internal/runtime/runtime.go - Added ExitOnErrorJSON() for JSON error output
• integration/cli_contract_test.go - Added tests for JSON output

Backwards Compatibility

✅ All existing behavior preserved - without --json, commands output exactly as before.

Testing

# All 188 tests pass
go test ./... -short